I have to say that VA is wacky and service-connection percentages (and what they mean) is no exception.
The percentages are supposed to indicate 'degrees' of social and occupational impairment...however,
I'm pretty sure that veterans who are 'just plain ole' 100% s/c can actually be employed and have income. If you tack on other designations such as 'individual unemployability' (which you can get at a lower than 100% rating) or 'permanent and total' status, I'm not sure about whether you can work and earn over a certain amount per year.
I KNOW for a fact that I have had plenty of veterans at 70% s/c for mental disorders who were working full-time jobs with high pay and excellent benefits.
So, the percentage does not seem to correspond directly with actual degree of impairment in percentage terms. To me, they seem to operate more as ordinal designations (essentially, no disability = 0%, minimal/low disability = 30%, moderate disability = 50%, high disability = 70%, and very high disability = 100%). VBA has also published lists of 'criteria' for each of the aforementioned percentages disability but they are not disorder-specific...i.e., they just have an omnibus set of 'mental disorder' criteria regardless of whether the veteran has bipolar disorder, depression, PTSD, adjustment disorder, you name it.
Another tidbit is that 'service connection' can just mean that the mental disorder was incurred (began or significantly worsened) during the veteran's time of service--not necessarily that the mental disorder was CAUSED by an event that occurred during service. So, if someone was in the Army from ages 18 - 38 and their bipolar disorder age of onset was, say, 20 years of age (while in service), the bipolar disorder would be service-connected because of WHEN it had its onset, not WHY.
It's wacky and often not at all in line with what the public would assume or, hell, with what most veterans (who haven't navigated the system) would assume.