This attribute indicates a y-axis coordinate in the user coordinate system. The exact effect of this coordinate depend on each element. Most of the time, it represent the y-axis coordinate of the upper-left corner of the rectangular region of the reference element (see each individual element's documentation for exceptions).
A <coordinate> is a length in the user coordinate system that is the given distance from the origin of the user coordinate system along the relevant axis (the x-axis for X coordinates, the y-axis for Y coordinates). Its syntax is the same as that for <length>.