Older systems (and some still call it this) had day and night mode... Day
was disarmed with 24hr zones (if any) still armed and Night was normal Armed
mode...
You also have Away (armed) and Stay (armed) modes... Think of it as Night
Away and Night Stay... Not really a difference between them, they are
complimentary to each other.
Night arm would typically be Armed Away though... as opposed to stay...
(didn't really see alot of Night Stay modes on the older panels)
Since we all seem to be guessing here, my guess would be that "night arm"
would equate to "double stay arm" or "instant arm" (perimeter on, motions
off, delay doors changed to instant for that cycle...)