It's a sub id, you can find it here on steamdb

Though I have the F2P licence on my account for Frontline Tactics I can't enter this giveaway due to "Missing Base Game", and I assume that given there are no entries nobody else can either. This sub looks to be the same but with the addition of the F2P base game which in theory shouldn't have the issue this one has. Not sure if it can be changed now, but I hope this helps.