Since only using psave=, I have NOT tried to do a pristine boot without it, save a session, and then review if the SAVESPEC is placed such that it is found on subsequent boot. I will try the current 'ScPup64-21.04+2-T.iso' and report back here.
Edit: This report verifies your indicators.
The following shows a simple occurance: I merely downloaded and booted the ISO in a single step, broken down as follows (in case someone wants to make a video for PUP/DOG booting):
Downloaded 'ScPup64-21.04+2-T.iso' to /BOOTISOS on SG2D USB stick
Booted USB
Selected "ScPup64-21.04+2-T"
Pristine boot (Enter key only at ScPUP's Menu)
Filled-in FirstRUN with locale changes
Updated Package manager
Setup SAMBA
Tested SAMBA
Minor desktop changes
Shutdown processing
chose EXT4 for folder session save (this is NOT on boot USB)
change the pre-selected folder name to /Sessions folder
Shutdown
Rebooted USB and selected same ISO
At ScPup64 Menu screen, merely hit the Enter screen (same as before)
ScPup64 booted, found its save-session showing [SAVESPEC]
Everything, as before my prior shutdown, is present...ready to go!
Thanks @gyrog for this eye-opener. This is an even better game changer for PUP/DOG use!!!
I, now, no longer need to ever use "psave=" with this built-in ability. Puppy Linux booting has just gotten so much easier to boot and manage. This replaces all need for use of Frugal with this knowledge.
GREAT feature thanks to you and the PUP+DOG developers who made these changes for our benefit.