I was confused too when I got my truck and wanted to test the coolant. The PO had replaced the coolant, but I didn't know with what kind. Ford has their own formula for protection from cavitation that can only be tested w/ Ford test strips. NAPA also has their own different formula for cavitation protection and can only be tested with the NAPA test strips. Trying to test the system with the wrong brand test strip will cause a false reading.
There's been lots of debate over which NAPA, CAT, John Deere, Ford products are better than the other. But my uneducated opinion is as long as you're running something and monitoring it on a regular basis, any of the products above will be fine. I just flushed my system and went w/ the NAPA system b/c of availibility and pricing issues. This is just the conclusion that I've come to for my own understanding, much smarter people on this board can probably give you a very educated opinion.