When it's filled. If you click on it, perhaps a message that says "this position is occupied by Extra1" or something.
It's a small thing, but eventually you will get to recognize names sitting in shore batteries not firing, or sitting in CV 5" guns while Buffs from their main account dive bomb the carrier.
Not that it'll stop it, but at least it will, perhaps, save some wasted time typing "who's in the battery and why isn't it firing?" in country chat, or asking in ranged who's manning the defenses of a field or group...