Ok, so for that particular set (since it’s 20 yrs old) they couldn’t pull the inventory from Lego.com, so it is being pulled from Rebrickable as a backup. Other sets that Lego does have the inventory for, are pulled from Lego.com and have an option to pull instead from Rebrickable.
Wait, people care about sets that aren’t 20 years old…?
That tab is for an individual set. Erin is looking to curate her own inventory and keep track of it across multiple sets.
OK, in that case, I’m just going to stop talking, because that’s a level of personal inventory management that I’ll never even attempt to handle. You kids have fun; sorry for making lame jokes and pointing fingers without even understanding the question!