Zip disks presented themselves with drive letters higher than B (usually D: assuming you had a single hard disk). However, some (all?) Zip drives could also accept legacy 3.5" floppies, and those would show up as B.
You're confused and you're thinking of the LS-120 SuperDisk. On some machines, it could be setup to appear as A: or B: when a 3.5" floppy was inserted.
Zip drives were never compatible with 3.5" floppies, and always were enumerated using the first available external storage letter (ie, D: in typical machines).
Zip disks presented themselves with drive letters higher than B (usually D: assuming you had a single hard disk). However, some (all?) Zip drives could also accept legacy 3.5" floppies, and those would show up as B.