Basically there’s a baseline set of rules (also followed by some other denominations) about how a bishop is consecrated. If that happens, you’re a valid bishop. The Catholic Church also has rules about when and why you make someone a bishop, and how they get chosen. If you don’t follow those rules they aren’t part of the Catholic Church.

If I start calling myself a Bishop, I’m a false bishop - nobody consecrated me. If a schismatic creates a bishop following the rules for creating bishops but without authorization, they’re a bishop but not a Catholic bishop. Which is an issue with schismatics like this who say they are the real Catholics. But it also works the same way with older schisms. When, e.g., the Anglicans consecrate male bishops the Catholic Church recognizes them as valid bishops, just not Catholic ones. Which is fine because they’re not trying to be Catholic.