PixelsPerPt is the window's physical resolution. It is the number of pixels in a single geom.Pt, from the package.

There are a wide variety of pixel densities in existing phones and tablets, so apps should be written to expect various non-integer PixelsPerPt values. In general, work in geom.Pt.

The value is approximate, in that the OS, drivers or hardware may report approximate or quantized values. An N x N pixel square should be roughly 1 square inch for N = int(PixelsPerPt * 72), although different square lengths (in pixels) might be closer to 1 inch in practice. Nonetheless, this PixelsPerPt value should be consistent with e.g. the ratio of WidthPx to WidthPt.