IMTU, I'd tweak Aramis' post very slightly to reframe X as:
X means no guaranteed facilities at all.
That allows X to have a consistent interpretation for your intrepid adventurers - you can't count even on a place to set down the ship - but have many interpretations:
* Unsurveyed - completely unknown. Could be a TL-J world that'll refurb your quaint ship just for nostalgia, or could be a toxic hellhole that will dissolve bonded superdense.
* Unpublished - for whatever reason, the IISS and/or TAS don't have data, or don't want to share it. Shades of...
* Uncertain - the class B on a Balkanized world fell into the hands of a fascist regime who have closed it out. That news made it out, but the fact that there are many class C starports available in system wasn't.
* Unavailable - a perfectly serviceable port was shuttered to civilian traffic by the Navy, Scouts, etc
* Actually absent - not even a marked slab of rock. Maybe it's a waterworld with no platforms, of a desert world with only sand dunes and ice caps.
While Forbidden + Red Zone does convey the meaning of some of those, X captures the important data at the start of the UWP - Travellers can not assume any services are available.