Question

Can all combistible materials be considered as fuel?

Answer 1

No, all combustible materials can't be considered as fuel.
A combustible substance is the one which can burn. It does not necessarily mean that it would produce large amount of heat without leaving any residue. Thus all fuels are combustible but not all combustible substances are fuels.

Answer 2

Combustible materials are materials which can be burnt in precence of oxygen or air. Thus if a substance is Burt it produces energy in the form of heat I.e. can be considered as fuel.