AFCP stands for AXS File Concatenation Protocol, and is a poorly designed protocol for appending information to the end of files. This can be used as an auxiliary technique to store IPTC information in images, but is incompatible with some file formats.
ExifTool will read and write (but not create) AFCP IPTC information in JPEG and TIFF images.
See http://web.archive.org/web/20080828211305/http://www.tocarte.com/media/axs_afcp_spec.pdf for the AFCP specification.
Tag ID Tag Name Writable Values / Notes 'IPTC' IPTC - --> IPTC Tags 'Nail' ThumbnailImage N 'PrVw' PreviewImage N 'TEXT' Text N