What determines which fields are available in the views relationship options?
When creating a view, you can use Relationships, choosing from a list of available fields. Are all created fields supposed to be available for relationship in any view? It seems that some fields are available and some are not.
Example: I have Students and School as my Content Types. Students uses a Node Reference to School. When creating a view, I use Relationship, and a bunch of fields are there to choose from. As expected, School is there, because it is referenced in a field. But there are a lot more fields available. Why are those fields available? What criteria do fields fill to be available to use?