PrF UG classes constraints
Protege-Frames builds relationships (e.g., between classes and instances, slots and slot facets). However, these relationships may be insufficient to capture all the constraints that hold among the frames of the knowledge base. Typically, Protege-Frames does not offer a way of relating the values of different slots attached to a given class, nor does it help in relating the slot values of different instances of a class.
For example, in the newspaper project, one might want to state that editors should have a higher salary than the employees for whom they are responsible (i.e., a constraint that involves the Editor and Employee classes and the salary and responsible_for slots).
The Protege Axiom Language (PAL) extends the Protege-Frames knowledge modeling environment with support for writing and storing logical constraints and queries about frames in a knowledge base. PAL is a plugin toolset that comprises engines for checking constraints and running queries on knowledge bases, as well as a set of useful user interface components. PAL's primary purpose is to support the definition of arbitrary logical constraints on the frames of a knowledge base.
Information about constraints can be found in the PAL section of the Plugins Library, including a quick reference guide and comprehensive documentation.