LM-31 is a catchall. It can be autobanner ban, payment issue ban, or GM initiated ban. LM-11 is temp. LM-17 is perma. Both LM-31 and LM-11 can turn into LM-17 after a period of time(usually the length of suspension, but sometimes it happens later). Sometimes you'll get a perma and it'll stay LM-31 forever. LM-31 is not a guaranteed perma, if it's payment issues you can resolve it with support and if it's a suspension it'll go away on it's own(usually a day late, if they say 72h it's going to be 4-5 days from the time you got disconnected).
You will receive an email saying how many days if it's a temp, they claim to have a review period where it may turn into perma at the end of the temp ban, but in recent years a temp seems to always stay a temp. The email is the best way to know, since their use of the error codes is inconsistant.
If you exploit, they'll also straight delete the character from DB. You can log into SE account and it'll no longer show the char name under your POL account. That's always perma.