The flag is set in a maps [Basic] section and takes a value of 0,1,2 or 3 (Default is 0). When set to 1 or higher, the player will get a loadout screen allowing them to select units that they will recieve as dropship reinforcments during the mission. The value specifies the number of dropships that will reinforce the player and is limited to 3 as this is the maximum number that load screens exist for. Each dropship can be loaded with up to 5 units. Although undocumented, the player progresses to the game by pressing the space key after selecting his reinforcements.
The feature was never completed for TS and was left in RA2 as none functional. In TS there is a bug that prevents the cursor showing on screen making the feature difficult to use and in RA2 the reinforcements cannot be made to arrive at all in game. This can be set on multiplayer maps and all players will get the chance to select units, irrespective of their side actually getting a suitable reinforcement trigger or not. The game will also timeout if any player is delayed at the screen, something quite likely with no cursor or explaination of how to progress.
See also: AllowableUnitsAllowableUnitMaximums