It's possible they mean the other lid you can get for doing slow cooking (not the one that locks in place, but more like a regular type of pan lid).
I'm guessing one in the fridge and one in the cooker, so once you've cooked multiple days of meals you don't have to have the same thing continuously.
Just so.
Um, to seal the pot that's in the fridge?
The lid seals the entire Instant Pot, not the insert (which is the thing that would go in the fridge).
They make one to seal the pot that goes in the fridge, too. I don't think it is used for cooking at all. I havn't bought one yet, but it is on my list.