Premium member only
Cache owners can mark a geocache as Premium member only, often shorted as PMO. This mean that only users with a premium membership will be able to see these caches online or in various apps. Users without a premium membership can log PMO-caches online if they have found the cache and signed the logbook for example together with a premium member. Project-GC's Search page can supply the link for a basic member to log the visit of a PMO-cache.