Initially explored systems are completely explored.(only few)
Without the need of having been there we can always look "into" that system ingame.
Initially unexplored systems are.. unexplored(and in laaarge numbers)
Who enters an US(unexplored system

If he does NOT circles the ship to center those "unknown" object(to finally scan and identify it) for him the system will stay marked "red"(ingame!) in spite of having been in that US.
From that moment, the sun is identified those ingame marking changes to white.
Using a discovery scanner(hold the corresponding key/button until scan finished) will just MARK objects around you as "unknown" possible targets.
You have to get close to any of them(whilst targeting it!) to scan them automatically as well.
If Stations exist they will be shown as targets even not in range.
Suns are the only objects which will be shown in US on the ingame-system map from the moment you entered that system. Bad luck there are no targets for them, you have to "guess" where they may be. And you know, they can be far, far, far.. away!
For other objects you would have to cross a system with active scanner again and again (or you rob Diso's bank and get a system-wide scanner aka ADS).
Whew.. i lost the plot..
So as you decided to have ONE 4 tier tag (unknown/unexplored/part./full) it will reflects the actual system view state in your Roguey(TM)-universe.
lets have a look at each "tag":
unknown
- default value until someone decides to change it
initially unexplored
- maybe some data was put in like population/government
- no stations were added, no planets
partially explored
-all stations were added, some planets
-due to not having those system-wide-scanner(ADS) not all suns/planets were found/explored
Fully explored
-every object has been found AND added to system map
Problem is.. some systems are so big (4 suns, more than 40 planets..) they wont even fit.
As proposed somewhere in this thred earlier, the EXACT duplication of any object is unnecessary.
Things which seems to be important to me for players:
-Stations
-Blackmarkets whereabouts
-Type of stations(outposts are .. overcrowded sometimes but less protected by system security)
-Stations distances from entry point(some system's entry point is NOT where we expected)
Therefore I would suggest to add a locked thread for "rules" or hints regarding the use of adding data to the systems.
How to find coordinates, when to use which tag..
Because the above written defintions are NOT definitions.. they are my guess about their meaning and how they could be used.