Game Modes System

In Red Alert 2 a system of customizable 'Game Modes' was introduced. The parent INI (mpmodes.ini) configures the available modes, and under what player arrangement each mode is available, while also specifying which INI to load for the main characteristics of each respective mode. New modes can be defined, but they must fall under specific hardcoded categories, which will be detailed below.

Any multiplayer maps that list a mode in GameMode= under [Basic], will be shown to the player in the game lobby, when that mode is selected as a map filter. Unfortunately, the current crop of map editors don't handle new game modes perfectly, and they can be lost when re-saving a map using them.

Getting Started

The first thing to understand is that game mode INIs, are like selective rules(md).ini overrides, they cannot override art(md).ini or ai(md).ini in any way. This means you can fiddle with the unit properties, or the ownership of buildings, but you can't add new cameos, nor can you tell it to load new voxels in place of existing units.

When you play a specific game mode in RA2, the game will load the rules(md).ini first, then it will load the game mode INI contents, changing existing values or adding new tags as specified. For example, in a game mode, you may have just a single entry:

[E1]
Strength=200

In this case, the difference between a normal game and your new game mode is that the GI has 75 extra strength.

If you look at the MP mode ini files that come with Yuri's Revenge you will see how the various game modes are defined. For example, mpmwmd.ini (Megawealth) contains the ini override settings necessary to disable the Refineries and Ore Miners, and change the prerequisites of the other structures in the game (which normally require a Refinery before they can be built).

Finally, it seems that all MP game modes must have the following entry:

[MultiplayerDialogSettings]
AlliesAllowed=yes    (or AlliesAllowed=no if it is a Free For All mode)

Beyond that, any override settings are up to you. Just make sure you follow the normal procedures for rules entries. It is a good idea to base your mode file(s) on similar mode files already present.

Note: You cannot define a weapon in the main rules and then give it to a unit in a mode file. Doing so will cause an Internal Error in-game. All weapons must be parsed by a unit in the main rules file.
To do this, create a dummy, non-buildable unit in the main rules first and give the weapon to that unit.
There may be other situations like this that you will have to look out for.

The Multiplayer Mode Definition File (mpmodesmd.ini)

This file defines all the multiplayer game modes that are present in the game.

There are 5 multiplayer mode sections in mpmodesmd.ini.

[Battle]	This is a standard battle mode.
[ManBattle]	These modes may only be played in network and internet games. AI players are not allowed.
[FreeForAll]	Players may not ally with each other.
[Unholy]	The Unholy Alliance mode is in this section.
[Cooperative]	The Cooperative mode is in this section.

It is not known if all modes have to be defined in their appropriate section. For example, 'players may not ally with one another' is a setting defined in the Free For All MP mode ini file so the Free For All mode may not have to be defined in the [FreeForAll] section for it to work. However, there is no reason not to define each mode in its appropriate section anyway.¹

Let's take a look at the 'Free For All' mode.

2=GUI:FreeForAll, STT:ModeFreeForAll, MPFreeForAllMD.ini, standard, true

It is a free for all mode and hence it is defined in the [FreeForAll] section.

The first item defined in this mode is its slot position in the list of available modes when you choose the mode and map you are going to play on. The 'Free For All' mode is the second one in the list because its index number is '2'. Look at the other modes defined in mpmodesmd.ini. Notice how the mode whose index number is '3' is the third one in the list of modes in game.
Never leave out an index number. If you define a mode with index '15' you must have first defined a mode with index '14' (if you miss a number out, the game mode will still be playable but the selected mode will revert back to mode '1' after playing).

The second item defined is the name of the multiplayer mode. In the string table ra2md.csf, the entry 'GUI:FreeForAll' (in the GUI section) will have a value of 'Free For All'.
This is the title that is displayed in the graphical user interface (GUI) of Yuri's Revenge.

The third item defined is the description of the multiplayer mode. This description appears in the bar at the bottom of the screen when selecting a multiplayer mode. In the string table ra2md.csf, the entry 'STT:ModeFreeForAll' (in the STT section) will contain the description of the 'Free For All' mode.

The fourth item defined is the name of the MP mode ini file . MPFreeForAllMD.ini contains the Free For All rules 'addendums'.

The fifth item defined is the type of map that this mode may be used with. Map types include 'standard', 'teamgame', 'megawealth', 'duel', 'meatgrind', 'navalwar' and 'cooperative'. The type(s) of a map is set in the map editor and a map may be more than one type.
'Free For All' may be used with any 'standard' map.

The final item defined is whether or not maps created by the random map generator are allowed. 'true' means they are allowed, 'false' means they are not allowed.

¹ ↑ I believe the different headings invoke certain logic specific to certain game mode types. For free for all it makes the AI attack other AI's as well as the player and UnholyAlliance make the game give all available base units. You might want to re-write this section with that in mind - Blade

MP mode-related Bugs

There is a strange bug with MP mode ini files that can cause some undesireable rules overrides.
There are 3 known cases:

Pip= & OccupyPip=

Let's take the GI as an example. We are going to edit the GI as follows:

[E1]
Strength=200

When we play in-game we will find that both the colour and shape of the GI's pip (when inside a transport or occupying a building) have been changed or 'messed up'.

The same applies to any infantry unit you modify.
To fix this problem, you must do the following:

[E1]
Strength=200
Pip=white
OccupyPip=PersonBlue

You must restate these pip settings for every infantry unit you mention in the MP mode ini file.

PrismSupportModifier=

When you modify the [General] entry, the PrismSupportModifier flag gets changed. It causes a supported Prism Tower to do instant-kill damage to its target.
To fix this problem, all you have to do is restate the flag:

[General]
SecretInfantry=SNIPE,DESO,TERROR,YURI,MYNEWUNIT
PrismSupportModifier=150%

InfantryType + DeploysInto = IE

Also you should be aware of the really awkward IE that is listed as #14 in the known causes for internal errors list (redefining [in any way] an infantry that has (Un-)DeploysInto= set causes an IE when a human player scrolls the map to the AI war factory).

Image=

Using Image to override or replace TechnoType image in game mode file can come with couple of unwanted side effects, listed below.

1. Any flags in the original art entry (the one referred to from main rules file) that are not not found and thus overwritten in the new replacement one, will be carried over. This is prone to cause display issues, for example when the two art entries have different amount of animations attached to them.
2. Some flags do not get overwritten even if they are specified in the new art entry. An example is Cameo, and possibly AltCameo too. (This would require more testing in order to gather a more conclusive list in regards to what works and what doesn't.)

Implications

These bugs create a rather worrying problem as they pose the question: "What other flags get buggered by MP mode ini files?" If you discover any more, please post them above.

In light of the above bugs, it is a good idea to avoid using mode files wherever possible, and keep as little code in them as possible. When modifying an entry, try to restate the entire entry's code, just to avoid this bug making an appearance.

This page is basing on the Multiplayer Mode INI Tutorial by Marshall