The second PET mode is a kind of inventory. It shows you what game
objects you have already picked up.
If you come across an object that you want to put in your PET, simply
click on it and drag it into an empty box. (In fact your PET will even
accept objects if you're not in Personal Baggage mode.)
You can also drag items out of the PET -- to put them in the Succ-U-Bus
tray or give to a Bot for example.