most of the definitions i've readily found are a little vague:
proof of claim A formal notice provided by the creditor of the amount owed and the nature of the debt.
however, it generally means (methinks):& show me the original contract that we have with each other.
a proof of claim can be an original invoice signed by the buyer, any valid contract signed by the two parties, etc.
anyone else whos knows better, please pipe in...
jon