Connecting the N-Tester and performing measurements

How can I connect the N-Tester BT to the Atfarm mobile app and measure with it?
How can I connect the N-Tester 2011 and N-Tester 1995 to the Atfarm app and measure with it?
Why isn't a certain crop or variety available for the N-Tester? Can I substitute it with a similar crop or variety?
What is the 3-digit N-Tester value and how to interpret it?
What factors can affect the N-Tester reading and which do not?
Do I need the Atfarm mobile app to use the N-Tester? Where can I find the variety correction lists?