Different versions of Ben Tennyson and the omnitrix.
There are many universes in existence where the omnitrix is real and Ben Tennyson is also real. Many Ben Tennyson's are our allies but at the same time many of them are busy protecting their own universes.
We the Teen Oracles possess Omnitrixes which can be picked up in Psi World that allow us to transform into aliens astrally. These omnitrixes also allow us to communicate with these other versions of Ben Tennyson.
It is suggested that when you pick up an omnitrix attend plumber training or begin training with a version of Ben 10.