"Big Six" means AA/DL/UA/FDX/UPS/SWA. Those are going to be the best airline jobs for most people, if you can get one. If you have at least 50 combat missions in a fighter and are a weapons school grad, you should hold out for one these.

The "tiers" are less defined. First Tier probably means Big Six, maybe include the other two legacies.

Second Tier is in the eye of the beholder. Jet Blue is probably the only airline which no one would debate as being second tier. AS, HI, NK, might also be considered second tier.

Third Tier would be all the other majors, not including ACMI

Cargo and ACMI are their own thing I guess, other than FDX/UPS which are light years better than other cargo outfits.

Regionals are regionals. Bottom feeders usually means mesa and go-jet, but those are some blurry lines as well.