I think you are talking about Consider If formulas.
so as the name implies, before call calls gets queued that particular skill group, ICM will evaluate the condition defined in if condition happens to be true only the the call will be queued that particular skill.
attaching screenshot for your reference, and as you can see in i am before queuing call to that particular Skill Group, i am checking if any agents are logged in to skill group or not. and if not i don't queue to that skill at all. you can use formula editor to create the conditional clause.
