This command computes the MD5 signature of all data parts contained in a given FITS file (including all extension data if present) and prints out the value on stdout. It can also update the main FITS header of a file if it already contains a keyword called DATAMD5.
This feature is meant to give a unique ID to a FITS file. It is a good solution to uniquely identify frames among a large list of FITS files that can be renamed or which FITS header can be modified. Most users will not need it at all, but people dealing with very large numbers of FITS files might want to evaluate it to solve frame identity issues.