Is it possible that the first sentence was a misprint?
I don't find the first one particularly meaningful.
The second, containing "unique within document", says to me that the following is true, where SN means "student number":
Document 1: SNs: 45, 75, 35, 19, ... (no duplicates
within this document)
Document 2: SNs: 45, 25, 17, 75, ... (no duplicates
within this document)
...
...
But notice that the SNs are not unique
over the totality of documents. (45 and 75 are duplicates.)