I think what @krispn means is the string being fretted as in playing a note rather than it being open.
When you are setting up and measuring the relief you tend to fret the string at the first fret and where the neck hits the body and check with a feather gauge how much relief you need in the neck.
General parlance is measuring from the fret to the bottom of the string at the 12th, to set saddle heights and string height, then at the first fret for filing the nut.
Measuring the string height without fretting a note is not great as in general unless it’s stupid low, or you have set relief as such it shouldnt buzz
the string heights will vary across the strings slightly being the norm, or not depending on your flavour, but they tend to