Vertical unit
A vertical unit is defined as half the height of one quarternote-notehead.
As a consequence staff lines are separated two units by definition. The location of a note on a staff therefore is as easy as possible; every diatonic note has an integer position: a B4 (center staff) has vertical position 0, C5 is 1, A4 is -1 etc.
Whenever you make an offset to a vertical position of an object it is measured in vertical units.