What is the closure property in this case

Question

There is requirements specification which has certain Completeness properties:

1. No "To be done" phrases
2. No references to non-existing items
3. No missing items
4. No missing functions
5. No missing products

The first two of these properties form a subset of the completeness properties which are called "closure" properties.

I am trying to understand why is that? The only thing I can think of is that these "work" with the existing items and are known (I could count the number of TBDs or wrong references), while I cannot know whether something is missing. Am I on the right track?