a mid advent = a worth of 6-9 09 rares. It's a unit of measurement, so if someone says a pet is worth 1 MA it's worth 6-9 09 rares. If it's worth 2 MA it's worth (6 times 2 --- 9 times 2) 12-18 09 rares and so on. The name comes from pets that were on the middle of the advent list (part of a list of the most valuable very rares on CS), and those pets themselves are worth 1 MA like the name says
A non is the same kind of thing but for the most expensive pets on CS,
1 non = 100-150 09 rares (usually 100)
1 Non can also be broken down into 10-12 MA instead as well
If someone says a pet is worth 2 nons, then it's at least 200 09 rares, or 20 + MA
It also works in fractions, for pets worth more than 2 or 3 MA but aren't worth a Non
0.75 non = (3/4 of a non) = 75-81 2009 rares, or 8-9 MA
0.5 Non = (half a non) = 50-75 09 rares, or 5-7 MA
0.25 = (one quarter of a non) = 25-37 09 rares, or 3-4 MA
The name came from 4 specific dogs, the non-coon, non-swirl, nonjewel and nontag, and those are worth 1 non each themselves
NA could mean not applicable like Hades said (or it might just be a typo)