Under Instruments, you manage the list of instruments available in your band. This list is what members choose from on their own profile, and it's what sheet music parts are matched against — so someone whose profile says "Clarinet" automatically sees clarinet parts in My sheet music.
Add an instrument your band doesn't have yet, or rename/remove one you no longer need. Removing an instrument that's still assigned to members or sheet music parts isn't recommended — reassign those first.