The PayPlanStatus is if the invoice has a payment plan on it. Infusionsoft generates a payment plan for most orders unless they are created onto the job table (unless they have changed this) directly. This isn't recommended.
Apparently that field has three possible outcomes that it could be although one isn't possible anymore
0 = Doesn't have a payment plan
1 = Has a payment plan
2 = Payment plan was removed (This status shouldn't be possible for any orders created within the past few years, obsolete 4 years ago at least)
You are correct with the Pay status it is 1 for paid in for and 0 for not paid in full.
CreditStatus I can't see any documentation on it. I can run some tests to try to see if this interacts with the credits from a subscription purchase however these tests will take a couple of days to complete and I won't be able to post back until next week.
For RefundStatus @Pav is correct with the status although there are two more possibilities:
0 = No Refund Issued
1 = Partially Refunded
2 = Completely Refunded
3 = Written Off
4 = Partially Written Off