What determines which fields are available in the relationship options? 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?
One thing that affects the choice of the fields that are available is the show box when you first create the view this allows the developer to refine the criteria that is being chosen.