Which SCSI instructions are used to build a bitmap of the changes between the first...

DIFF protocol - A vendor specific SCSI command which RecoverPoint uses to query XtremIO with in order to obtain a bitmap of changes between two snapshot sets.

RecoverPoint uses the output of DIFF command to read the actual data and transfer it to the target side.

References: EMC RECOVERPOINT REPLICATION OF XTREMIO, Understanding the essentials of RecoverPoint Snap-based replication for XtremIO, page 9

https://www.emc.com/collateral/white-papers/h14296-wp-recoverpoint-replication-of-xtremio.pdf