About Transaction of MyRocks

There are several transaction structs in MyRocks,

Rdb_transaction(Rdb_transaction_impl Rdb_writebatch_impl) TransactionDB Transaction

I know between the relationship of TransactionDB and Transaction, see the comments of Transaction:To use transactions, you must first create either an OptimisticTransactionDB or a TransactionDB.

howerver, what is the function of Rdb_transaction(Rdb_transaction_impl Rdb_writebatch_impl)? what is the relationship of TransactionDB and Rdb_transaction?