For vanilla space marines, certain unique characters changed the base rules for the space marine army so that the army's feel was more in line with the chapter's traits that the character was from.
For example, Lysander, who is an Imperial Fist special character, replaces the "chapter tactics" special rule (which allows space marine units to voluntarily fail morale checks -- a strategic regrouping) all units get with the "stubborn" special rule (units ignore negative modifiers to morale checks -- to represent their fierce resistance to impossible odds).
I propose one addendum: if one player takes a special character that changes the rules for his army (like in the example above), and another player is also playing the same army, then that second player may also benefit from the rules change.
So, if there were two imperial fist players, and one fielded lysander, both players could benefit from Lysander's special stubborn rule if the second player declared before the game started that he wanted to do that.
Mike