In the game, Chomp, his abilities, and the monsters all have tag affinities. Tag affinities are attributes that decide the strengths and weaknesses of the user. There are five affinities: fire, water, earth, dark, and light. Fire, water, and earth are the most common.
The fire affinity is strong against earth, but is weak against water and dark.
The water affinity is strong against fire, but weak against earth and dark.
The earth affinity is strong against water, but weak against fire and dark.
The dark affinity is strong against all of the other affinities, but leaves the user vulnerable to damage.
The light affinity is just the opposite. It causes little damage, but the user doesn't get much damage in return.
Chomp begins with the fire affinity, but as his later forms are discovered and unlocked, he can become any of the five.