As people might experience, the AP 4-pot calipers seem to get really hot when used properly which means the seal-life will decrease pretty rapidly.
The outer (dust)seals oftenly burn off which means they need to be replaced.
Not replacing these on a road car means dirt will get into the caliper and will destroy the inner seals after which the internals will be destroyed.
These seals are special High(er) temperature versions of the normal outer seals. They last longer than the original seals which are to be found in the normal 4-pot refurbishment kit.
Each kit contains 4 seals to upgrade 1 brake caliper.