Q:

Delete Rules for Many-to-One Relationship

Apple's documentation of Relationship Delete Rules is simple and clear. But it only talks about One-to-Many relationship(Delete Rules for One-to-One relationship are easy to infer). It's not clear what these rules mean for Many-to-One relationship. So let's clarify them here.
We use the Employees-Department example used in Apple's documentation. Though the real-life implications may be ridiculous of these rules applying to Employees-Department relationship, we, as programmers, are only talking about their logic implications here.

Deny
If there is an object at the relationship destination, then the source object cannot be deleted.
For example, if you want to deleted an employee, no matter whether there are still other employees in his department, you must ensure that the department is first deleted otherwise the employee cannot be deleted.
Nullify
Remove the source object from the inverse relationship of the object at the destination.   (See @bshirley's concise explanation)
For example, if you delete an employee, remove him from employees relationship of his department. This only makes sense if the number of remaining employees of the department is larger than the minimum count required, or if you ensure that you add new employees for the department before the next save operation.
[Question: If it is the last employee, will the department's employees relationship become empty set or null?]
(Answered by @TechZen: A to-many relationship always returns a set object. It is never nil. If there are not objects on the other side of the relationship, the set is empty.)
Cascade
Delete the object at the destination of the relationship.
For example, if you delete an employee, delete his department at the same time, even if there are still other employees in the department.
(Usage Caveat: It usually causes a "a daisy chain of deletions across the entire object graph", as described by @TechZen in his example.)
No Action
Do nothing to the object at the destination of the relationship.
For example, if you delete an employee, leave his department as it is, even if it still believes it owns that employee.

The meanings of Delete Rules for Many-to-Many relationship can be inferred from here.

A:

These are the delete rules for all relationships (not attributes).  They are applicable for to-One or to-Many relationships.

Nullify - if you delete an employee, the reverse relationship is set to nil, if it was 1-to-1 then literally, in this case department's employees is reduced by one
Cascade - if you delete an employee, it's department will be deleted.  The Department will follow the delete rule on all it's properties, 1) if employees delete rule was Cascade, all employees would be deleted by this action; 2) if employees delete rule was Nullify, all the employees would be "stranded" without a department

