APCS is a set of rules governing calls between functions in separately compiled or assembled code fragments.

The APCS defines:

Code produced by compilers is expected to adhere to the APCS at all times. Such code is said to be strictly conforming. Handwritten code is expected to adhere to the APCS only when making calls to externally visible functions. Such code is said to be conforming.

The APCS comprises a family of variants. Each variant is exclusive. Code that conforms to one variant cannot be used with code that conforms to another.