The User-Supplied identifier must be normalized into a Claimed identifier, which is just a fancy way to say that the identifier supplied by the user is transformed into a standard form.

Optionally, CHECK constraints can be defined on the pdbxml table to ensure that the four-character PDB identifier conforms to the PDB standard.

In the humble URL, we have (in theory, at least) a unique identifier for everything ever published on the web, which includes any apps published on standard websites.