I have read about The OpenPGP trust model but I don't see any mention of the specific signature types. Are all signature types (
sig3) treated the same by tools such as
gpg or the PGP Pathfinder?
Jens has answered this quite well. Basically, the signature levels are defined in the man page under
sig refers to cert level 0, which "means you make no particular claims as to how carefully you verified the key." This is the default when signing a key. They run up through levels 1 and 2, to the highest level,
sig 3, which "means you did extensive verification of the key."