I assume it would be anything that would come up in the DLC tab. So in League's case, since you can't buy something in-game through Steam in some way (unlike other F2P games I've played on Steam), you're still SOL for that.
Aw so you couldn't add 'LoL cash' through Steam? For example, Planetside 2.
Is that true though? If you have bought an item through a F2P game, does it permanently add it to your library now?