Issue types, schemes, and issue type schemes
Issue types classify work and drive which workflow, screen scheme and field configuration apply. The standard hierarchy is Epic → Story/Task/Bug → Subtask, although which type sits at the epic and subtask levels is configurable. An issue type scheme is an ordered list of issue types plus one default issue type, and a company-managed project uses exactly one issue type scheme. The default is what the generic Create button preselects.
Deletion traps dominate this domain. An issue type cannot be deleted while any issue type scheme references it or while issues of that type exist; you must first replace it in every scheme or migrate the issues. Editing a shared issue type scheme changes every project associated with it, which is why adding a niche type usually means creating a new scheme for the few projects that need it. Team-managed projects hold their own local issue types, so creating a global issue type does not make it available there.
The other trap is confusing the two similarly named schemes. The issue type scheme says which types exist; the issue type screen scheme says which screens each type uses for create, edit and view. A question describing different screens for Bugs versus Stories is about the screen scheme.
Worked example: you need a Spike type visible only in Project A. Create the issue type globally, create a new issue type scheme containing the existing types plus Spike, associate it with Project A only, then add Spike to the relevant screen schemes so it can actually be created and edited.