Evidential is an adjective that means serving as evidence. The receipt for the stolen blue suede shoes would be evidential proof of what you paid for the shoes when you bought them from the Elvis estate.
Often used as a legal term, evidential is sometimes paired with the words “proof,” “burden,” or “hearing.” Like many legal words, this one comes from medieval Latin (in this case, evidentialis). Lop off the last two letters and you have the English word. If a Roman took an Englishman to court for lopping at the words in his language, though, he’d need to show some evidential proof that the Englishman was guilty.