SCSI tape question

LOTS of SCSI devices violated the standard protocol in interesting ways. My understanding is there are two classes of SCSI drives. One class used a scheme that was quite like old 9-track tapes, so that there was essentially no formatting that wrapped around the user data. So, there would be a physical gap with no magnetic transitions between the written user data blocks. There would be a CRC of some sort at the end of the user data. Higher-end drives wrapped the user data into a more complex format, so each block would have a block size and a sequence number at the beginning, and maybe a trailer of the same info with the CRC. Often in these systems, there would be NO physical gap. This increased data capacity, but made it impossible to rewrite a single tape block. The file mark was just a special block with minimal or zero-length data packet.