Roles

In RTS games this usually exists in a form of a sub-faction that specializes in certain style of play, commonly divided into something like land, air, and support).

In most other games this exists in form of character classes, especially those that are assumed per mission, map, or such.

Commonly each available role has capabilities the others don't have, which makes choosing different classes in multiplayer better than everyone choosing the same.

For now, this should not be used in games better described as class-based.

In terms of realism, this implies you're some middle-level commander instead of the supreme lord of all, thus you're in charge of specific company (e.g. mobile armor, infantry, artillery, navy, etc.) or even lower down the chain of command (e.g. in command of a fireteam, vehicle crew, etc.).

Related:
* Class-based