Is there a guild for the fields that Product download exports as a CSV?
Some are covered in https://www.ecommercetemplates.com/help/admin-products.asp
but the User Manual doesn't cover them and I'm wondering how exactly pSell works.

pSell is the sell tick box (as seen on products in the admin if you are not using stock management) for each product.
If you have stock management turned on, then you can leave it blank as it's not used.
If you are not using stock management, then set it to 0 or 1 (numerical value)
0 would be the same as unticked in your admin and 1 would be the same as ticked.

If you do not want to use that column, you can just remove it from the csv file.

Thanks,
David