In a NON-BLOCKING merge, the most recently committed data from the RLV store is written to the IQ main store to create a new table-level version of the RLV-enabled table. This new table-level version combines the previous table-level version plus the changes from the RLV store (the in-memory changes from committed transactions). Uncommitted transactions may reference snapshot versions on the pre-merged RLV store. These fragments are held in-memory until the transactions terminate.
For BLOCKING merges the scenario is much simpler. There are no uncommitted transactions referencing snapshot versions on the pre-merged RLV store, nor are there any data changes happening while the merge is running. Hence, when a BLOCKING merge completes, there is only ever a single, empty RLV table fragment.