Although any member of a crew, provided they are capable, can take part in a fight regardless of their occupation, a combatant's specific purpose is to fight for the benefit of their crew. A combatant can be anywhere on a crew's strength hierarchy: some are officers of a crew and are considered to be their top fighters, while others are relatively average fighters who serve as foot soldiers or guards. The Big Mom Pirates, as a Yonko crew, have combatants who are nowhere near as strong as the top members but are still fearsome pirates in their own right and can easily deal with most fighters they come across.
Combatants have a massive variety of abilities, with many being swordsmen, gunfighters, or hand-to-hand fighters. They should not be confused with snipers, who specialize in long-range shooting and weaponry. Assassins can be similar to combatants, but their job is to specifically target and kill people for a variety of reasons while combatants simply fight against (and occasionally kill) people who are enemies of their crew.
The Big Mom Pirates organize their many combatants into ranks named after chess pieces, with the ranks of Bishop, Knight, and Rook being known to exist.