The World of Warcraft macro system has a way of setting optional modifiers in your macros, like [mod:alt] or [mod:ctrl], which lets you basically combine a few different macros into one.

If you check one of those boxes in the WoW Macro Library to mean that they are "allowed" for this macro, then when you use a Named WoW Macro Action for this macro the boxes will be available for checking. The macro itself "allows" the optional modifiers, then the Named WoW Macro Action "uses" the optional modifiers. This lets you select one of the "few different macros in one".

This is basically how "FTL" is done -- one macro is used to determine which guy to follow or assist, using the mod and nomod switches that are part of WoW's macro system.