Difference between RBK753 and RBK753NAS

Hi. I keep seeing two different model numbers for the RBK753 system. I understand that on one model there is an S which means it has one year subscription to Armor included. What I don't understand is that some RBK753 systems have "NAS100" after the "S". So, is a RBK753S-NAS100 the exact same as the RBK753S, or does it have some NAS features?

Thank you.