Retail FW Updates
There are 3 types of Retail System Updates.
for Release, which is the euqivalent of PS4s SYS (System) Update.PUP.
Then we have PRE-Install, the equivalent of PS4s REC (Recovery) Update.PUP. It's a complete package that also includes some additional system files not included in the standart Update.PUP. Thoes update files can only be used with the so called Rescue or Recovery OS.
And third the SD SystemData which is also the smallesd one of all three.