Quoted from LarryPounder

Also, another question. What are mercs and how they work?
(If it’s on FAQ, direct me, couldn’t find from there.)

mercs (or “mercanaries” or “ringer”) are players that are not on your team’s roster. If you want to play with a merc you usually need approval from your opponent to use him but this is different from competition to competition. In the Nations Cups no mercs are allowed at all, for example, and in fun cups you can usually use up to three mercs.