There are two bars below. The first is 600px long, the second 600pt.

On a screen, assuming 96dpi, 600px is exactly 600 device pixels long, and 600pt should be exactly 800 device pixels.

On a printer, 600px should be exactly 120mm assuming a normal printer and normal viewing distance, regardless of the resolution of the device, and 600pt should be a physical 212mm (to 3sf).