Probably both are the drive manufacturer’s recommended write speed for the disc in the drive.
To get/set write speeds for CD -R/W only drives to use 2A (bytes 28-29) & Set CD Speed, for DVD +/- R/W and CD/DVD Combo drives to use GetPerformance/Write Performance/End Speed & Set Streaming.
There are reported to be DVD drives that will not return a valid 2A. My old Plextor CD only writer (401240A) returns good Get Performance values. Andy Polyakov, author of growisofs, who knows a lot more than me says Set CD Speed is obsolete and to always use Set Streaming to change the speed. A value of FFFFh will set it to “optimum” where the drive’s bios decides the best write speed.
You do not send these commands during a write loop to check the speed.
For more details on 2A download the SCSI-3/MMC-3 (T10/1363-D, November 12, 2001) from the T10 website.