Mod for mod (also known as M4M) is a term that describes a trade between two or more creators. It requires each creator to mod the other's beatmap. 104m5j
This term is usually found on the forums where s review each other's maps, but may also be found on profiles and external sites.
Mod for mods have historically been favoured over regular mods since both parties benefit from this deal, though have become less common in recent years.