All supported metadata keys conform to a type.
User-defined metadata keys conform also to a type, based on the suffix of the key.
|any other suffix||EString|
The name of the metadata key is bound to the key type
Every key type has an associated validation rule to check values of the given type. There is also a rule how values are matched, e.g. against a search term when selecting some zettel. And there is a rule how values compare for sorting.