question about Ampere Frame

AF just means "Amp Frame". It is only used as a short hand way of describing the way a particular manufacturer has designated the frame size they chose to use for the trip value you are wanting. The trip value is referred to as "Amp Trip" or AT. Here is an example;

Siemens makes a line of breakers called the 3VL. They have several different Frame Sizes: 150A, 250A, 400A, 600A, 800A etc. If you need a 400A breaker, you can order that as a 400AT 400AF, but you can also order a 400AT 600AF, or even a 400AT 800AF breaker. All of them have 400A trip settings, but the 400AT 400AF version can never be replaced with a larger trip, because you are already at the maximum rating of that frame (400AF). If there is a chance that your application may require a larger circuit some day in the future, you can buy the 600AF 400AT breaker and if you need 500A next year, you can replace the Trip Module with a 500AT unit without exceeding the 600AF rating.

None of this has ANYTHING to do with the Short Circuit rating capacity. However, each manufacturer has economies of scale to contend with when making breakers. So when they make a 150AF breaker, there may be plenty of applications where 22kA is adequate. But in a 600AF breaker, MOST of the applications are likely ones where there will be at least 50kA available, so to avoid having to make and maintain inventory on 22kA 600A breakers that few people will buy, they only offer it in a 50kA version. There is nothing more to it than that.