Transactions involving RLV-enabled tables create row-level transaction snapshot versions. A row-level snapshot allows the transaction to commit a version of the table row, rather than a version of the entire table.
Row-level versioning permits concurrent DML changes to the table. (An update to one row may not block another connection's update to another row).
Open transactions hold versions which are accessible to that transaction. If a transaction is long-running, the server will hold the memory and disk space associated with the RLV and TLV versions until that transaction terminates. Even after a merge, memory for RLV versions will not be freed until the transactions with snapshots referencing these versions are terminated.