Assume a miner selects 10 transactions where their total gas is equivalent to the block gas limit (e.g., 6 million gas). But once the miner executed those transactions, he figured out the actual used gas by those transactions is less than the block gas limit (say e.g., 4 millions). Will the miner be able to select other transactions in order to fill the block?
In other words, whether the block gas limit is calculated based on the total gas provided by transaction or by the total used gas, since a user can send a transaction with 1 million gas, but actually their transaction can only use 500,000 gas.
I hope i made it clear!