gpg gets it absolutely right by not asking users this question by default. People should not be enabling this option.
Some background: gpg's --ask-cert-level option allows the user who is making an OpenPGP identity certification to indicate just how sure they are of the identity they are certifying. The user's choice is then mapped into four levels of OpenPGP certification of a User ID and Public-Key packet, which i'll refer to by their signature type identifiers in the OpenPGP spec:
- 0x10: Generic certification
- The issuer of this certification does not make any particular assertion as to how well the certifier has checked that the owner of the key is in fact the person described by the User ID.
- 0x11: Persona certification
- The issuer of this certification has not done any verification of the claim that the owner of this key is the User ID specified.
- 0x12: Casual certification
- The issuer of this certification has done some casual verification of the claim of identity.
- 0x13: Positive certification
- The issuer of this certification has done substantial verification of the claim of identity.
Most OpenPGP implementations make their "key signatures" as 0x10 certifications. Some implementations can issue 0x11-0x13 certifications, but few differentiate between the types.
By default (if --ask-cert-level is not supplied), gpg issues certificates ("signs keys") using 0x10 (generic) certifications, with the exception of self-sigs, which are made as type 0x13 (positive).
When interpreting certifications, gpg does distinguish between different certifications in one particular way: 0x11 (persona) certifications are ignored; other certifications are not. (users can change this cutoff with the --min-cert-level option, but it's not clear why they would want to do so).
So there is no functional gain in declaring the difference between a "normal" certification and a "positive" one, even if there were a well-defined standard by which to assess the difference between the "generic" and "casual" or "positive" levels; and if you're going to make a "persona" certification, you might as well not make one at all.
And it gets worse: the problem is not just that such an indication is functionally useless; encouraging people to make these kind of assertions actively encourages leaks of a more-detailed social graph than just encouraging everyone to use the default blanket 0x13-for-self-sigs, 0x10-for-everyone-else policy.
A richer public social graph means more data that can feed the ravenous and growing appetite of the advertising-and-surveillance regimes. i find these regimes troubling. I admit that people often leak much more information than this indication of "how well do you know X" via tools like Facebook, but that's no excuse to encourage them to leak still more or to acclimatize people to the idea that the details of their personal relationships should by default be public knowledge.
Lastly, the more we keep the OpenPGP network of identity certifications (a.k.a. the "web of trust") simple, the easier it is to make sensible and comprehensible and predictable inferences from the network about whether a key really does belong to a given user. Minimizing the complexity and difficulty of deciding to make a certification helps people streamline their signing processes and reduces the amount of cognitive overhead people spend just building the network in the first place.