When two transactions are merged, one is determined to be the 'kept' transaction and the other is the 'dropped' transaction. Any empty fields in the 'kept' transactions are copied over from the 'dropped' transaction and the 'dropped' transaction will be deleted. So, if the 'kept' transaction is uncategorized or has no payee, the payee and/or category will be copied over from the 'dropped' transaction before it is deleted.