Nestsets association: element / vertex

Is there a reason/use case to allow both? I feel like they both specify the same thing, and by allowing both, it increases the processing and validation code needed. For example, I would need to globally validate that all are either vertex or element to make sure things make sense.

The issue names no files, tests, or entry points. First clarify whether element and vertex should be mutually exclusive, then trace nestsets validation and processing to identify affected callers. Done requires an agreed behavior and corresponding validation or tests, but the current thread does not define acceptance criteria.

Written by the indexing model from the issue text.