This question was raised in the 'How to implement a secure IoT system on ARMv8-M' webinar, view all the questions in the round up blog post.
NSC stands for Non-secure Callable. A Secure region may be Secure or Secure, NSC. It is possible for non-secure code to make a function call into a Secure, NSC region. More information on memory partitioning can be found at: developer.arm.com/.../memory-system-and-memory-partitioning