The max is the largest cog compatibility. This has to do with the "geometry" of the derailleur's body. The idea is that you don't want the upper pulley bumping into the largest cog. It's impossible to say exactly what will work with a certain derailleur because there are other variables involved such as the derailleur hanger's exact position relative to the rear axle. I would expect your ARX to be able to handle a 28t cog.

The total capacity is the chain wrap capacity, which is how much chain the derailleur can "wrap" without the chain going slack. The formula for figuring chain wrap requirement for a given drivetrain is largest chainring minus smallest chainring plus largest cog minus smallest cog equals the total chainwrap requirement. I would expect your ARX derailleur to work with your proposed drivetrain in this regard as well.