AFCP is a specification which allows meta information (including IPTC) to be
appended to the end of a file.
It is a poorly designed protocol because (like TIFF) it uses absolute
offsets to specify data locations. This is a huge blunder because it makes
the AFCP information dependent on the file length, so it is easily
invalidated by image editing software which doesnt recognize the AFCP
trailer to fix up these offsets when the file length changes. ExifTool will
attempt to fix these invalid offsets if possible.
Scanning for AFCP information may be time consuming, especially when reading
from a sequential device, since the information is at the end of the file.
In these instances, the ExifTool FastScan option may be used to disable
scanning for AFCP information.