Hi guys. Random question ... can a faction re-expand to a system they retreated from? I have a feeling I know the answer but I want to confirm. Thanks, all.
It can be done - but if there's somewhere new in range, it'll go there instead. As far as I can tell, the order of priority for expansions is now (usually, surprises are possible):
1) System not previously retreated from, within expansion range, less than 7 factions
2) System previously retreated from since 2.4, within expansion range, less than 7 factions
3) System not previously retreated from, within expansion range, 7 factions {Invasion: requires no pending conflicts and suitable target faction for immediate War}
4) System previously retreated from, within expansion range, 7 factions {Invasion: requires no pending conflicts and suitable target faction for immediate War}
5) Investment: boost range and try again
6) Give up generating expansion states in this system for a while
Then, it will generally go for the nearest within a priority if there's more than one, though small difference in distances (e.g. 18.72 rather than 18.39) may be ignored.
You may see older documentation claiming a fixed (pre- or post-Investment) expansion range. This no longer seems to be the case, as I've seen unInvested expansions go a bit over 25 ... but also sometimes factions fail to reach somewhere closer than 25 without Investment.
Permit systems are much harder to expand into, but I don't have anywhere near enough information on how much harder as there aren't any in my region of expertise - it might be "they fit in between 1 and 2", "they fit in between 2 and 3", "they fit in between 4 and 5" or "when we said much harder we actually meant impossible".
A very small number of systems are completely locked to expansion - Shinrarta Dezhra, the three superpower capitals, Colonia and Ratraii, the detention centres ... maybe a few others. (These can be detected before trying to expand by trying to start a conflict between two already-present factions: in a locked system this won't work either)