It's not a "function". It's a condition (it's even in the name of "units matching condition"), and, as such, in any Action that has a condition part attached to it (given the action pertains to a unit!) it'll be there. It's also an option under just the plain old "Conditions" part of the Trigger.
Now, if you want to do something like: IF "Units match condition", "picked unit matches condition", etc, you want to choose the "if", "if/then/else" or "if/then/else (multiple functions)" from the drop-down menu.
If you meant something else, I'm terribly sorry. =P