The inventory is about to switch away from this (the active) item - to <paramref name="next" />, or to nothing when it's null. Return false to refuse and stay deployed (stash <paramref name="next" /> to finish a put-down and re-issue the switch), or handle it here - e.g. cancel an in-progress reload - and return true to allow the switch. Not called on forced holsters (death, removal, dropping). Base allows it.