KERML — Occurrences can be data values

7.4.1 (Kernel Overview) says

... data types cannot also be classes or associations, or share instances with them.

... data types classify things that do not exist in time or space ...

Clauses 8.4.4.2 (Data Types Semantics) and 8.4.4.3 (Classes Semantics) say:

The Type Base::DataValue is disjoint with Occurrences::Occurrence and Links::Link,

The Class Occurrences::Occurrence is disjoint with Base::DataValues

but Occurrence::Occurrence and Base::DataValue are not disjoint in the libraries.