The nitrate test for freshwater says:
This test kit reads total nitrate (NO3
–) level in parts per million (ppm
) which are equivalent to milligrams per liter (mg
/L) from 0 ppm
to 160 ppm
, in freshwater aquariums. The Freshwater Nitrate Test Color Chart is the appropriate chart to use for testing freshwater aquariums to which salt has been added."
Which, as said above, just means that you should use the freshwater test if you've added aquarium salt to the tank, as opposed to the saltwater test which is different.
So if you've added salt to the tank for whatever reason (treating Ich, for example) you can still use the freshwater test. If you're not adding salt, just ignore what it says and use the test.