I separated my 2T SSD iMac drive into Mojave OS and Monterey OS drives

That is fine, and I can restart between the two easily, when needed == but note that not only do I have the original Mojave drive on my iMac but also the Monterey drive [both APFS] but also some thing called Update -- any idea on what that is and whether I can/should eject it. So far not a problem. but I want to be careful. I was in the Monterey drive but restarted in the Mojave drive to do a Time Machine backup and it's still there.