Rank Fuse lets you pick one Bugmon as the base and feed other Bugmon into it as materials to raise its Rank. The materials you use get converted into Rank EXP, and any levels they had also carry over as regular EXP (so your base Bugmon gains levels too!).
This is the EXP needed to rank up your Bugmon. The higher the material's Bugmon Power, the more Rank EXP you get.
This is the EXP used for leveling up. Any training you put into the material Bugmon carries over to the base — nothing goes to waste.
You can use any Bugmon as material as long as it meets these conditions:
You can feed up to 100 Bugmon as materials in a single fusion.