From the docs:

If all of a managed object's relationship delete rules are Nullify, then for that object at least there is no additional work to do (you may have to consider other objects that were at the destination of the relationship—if the inverse relationship was either mandatory or had a lower limit on cardinality, then the destination object or objects might be in an invalid state).

Does someone have an example of this cardinality thing? What's this good for and what's important to know about this? (sounds very important...)