The motivation to create this site is to promote the uniform usage of signature notations and provide a namespace for them. Furthermore popular notations from other namespaces shall be documented here.
If you have an idea for a new uniform notation then you can submit it here. If a notation turns out to be widely used then it might be suggested for the official uniform namespace, the IETF namespace.
Not all keys have to be high security keys but you have to (reliably) know how secure a key is in order to be able to assess what use is acceptable for you.
You have to know what the key owner intends to use a key for (and what not). Key security alone may be enough information for encryption (and even then this is dangerous) but you may not interpret a signature more meaningful than the signer does.
known being up-to-date
You have to be reasonably sure that your version of the key is up-to-date.
To know the purpose limits of a key is not enough. You have to know what a certain signature is supposed to state about the signed data. This may range from as little as a timestamp to as much as a legal commitment.
The certification must contain all relevant information about the certified key (see above).
certification key security
How secure is the certifiying key (its main key, maybe differing from the subkeys)?
relationship to key owner
Does the key owner have a social relationship with the certifier (family, (long term) friends, acquaintance, stranger)?
What was validated?
fingerprint, name, email, additional information (UID comment)
Kind of verification
How have these claims been verified?
The OpenPGP standard provides all that is necessary: All these pieces of information can be put in certification notations. What we need are standards for expressing this information: human readable in the beginning, machine readable after official (IETF) standardization so that really usable trust models can be built on top of this by OpenPGP applications.
my project for supporting OpenPGP courses
Hauke Laging, hauke.lagingopenpgp-notations (add the obvious if your browser does not display a complete email address (due to a lack of CSS support))