I would imagine that if the software is licensed to the MAT then the licence covers all those within the organisation; whereas if the license is for a specific school then it can't be used across the MAT.
Just my thoughts
You have DataSec and SISRA which are both very good but not the cheapest. We use 4Matrix and find it the best value. After having used all three pieces of software, I can definitely recommend 4Matrix as the easiest to use and excellent value. IMHO