Are you talking about cert-digest-algo? It forces a specific hash algorithm, for example the SHA512 hash on this clear signed message. The standard is SHA1 (like in DPR's signed messages), but it is becoming obsolete. Some PGP programs use MD5, which is retarded. The GPG manpage does warn that you shouldn't force cert-digest-algo, because some PGP programs don't support SHA512 for message hashing, but I say fuck em. If your PGP program fails to verify a signed message because it doesn't support SHA512, you need a better PGP program. We shouldn't reduce our safety for the lowest common denominator.