The graphics engine determines how to style the PolyShortLine, and either sets the PSL_YMAJOR bit of the style field or clears it to 0. Therefore:

if(psl  usStyle & PSL_YMAJOR) {

Style it Y-major by adding pDC  usStyleRatioY to the error-term value upon each increment
of y as it is drawn.
}
else {

Style it X-major by adding pDC  usStyleRatioX to the error-term value upon each increment
of x as it is drawn.
}