Sonoran Acoustic Guitar: Action Adjustment by: Guitar Guru
It's the 6th, or Bass E string.
That is, if I understand your question correctly.
When adjusting the 'action', a luthier will fret one string at the FIRST fret and the LAST fret...simultaneously...and measure the distance between that string and the fret wire at the TWELFTH fret. A certain amount of 'relief' is necessary to prevent buzzing. And, it's usually the bass 'E' string.