Iron Test Kit Question

Greetings I'm trying to create a heavily planted freshwater tank and
read that iron deficiency is commonly a problem. Also that pH above 7
causes plants to be unable to use the iron that may be available in the
tank. So maybe I should buy an iron test kit. But before doing so, I'd
like to know more about what they actually measure. I mean, how meaningful
is the raw number the test kit will give? Any advice much appreciated.