From Marc Miller's
Agent of the Imperium:
... which covers the 16x8 sector region of space depicted in the classic Imperium poster, the Library Data supplements, etc. i.e. this:
So by that definition, "Charted Space" leaves off a lot of stuff we presume is pretty well, er, "charted", e.g. areas just rimward of the Solomani Sphere. And we know there are far-flung scouting expeditions by the Imperium as well as Coreward/Rimward expeditions by others. So a lot will be at least cursorily "charted".