AFCP (AXS File Concatenation Protocol) is a metadata format for image files. Its main uses are embedding IPTC data, and thumbnail images. It's not clear what AXS refers to.
It is designed to be used as a "trailer" that can be appended to a number of different file formats. It could potentially be used in other ways, and even has TIFF tag 34152 assigned to it. But it is not a self-contained format (it contains pointers relative to the beginning of the file), so such use would be problematical.
In most cases, the signature "
ASX!" or "
ASX*" appears beginning 12 bytes from the end of the file.