This is the coordinate space understood by the printer hardware. This coordinate system is typically measured in terms of the device's resolution. There is really nothing else that can be said about this space, as PostScript programs are typically not expressed using it. This is the coordinate system used by PostScript programs to describe the location of points and lines. User space is essentially the same as the first quadrant of the standard coordinate system used in high school math classes.
Coordinates are real numbers, so there is no set resolution in user space.
The interpreter automatically converts user space coordinates to device space. The transformation of user space coordinates to device space coordinates is done through the current transformation matrix. This matrix is a three by three matrix that allows the user to rotate, scale, and translate the entire user space within the device space. This is the source of a lot of PostScript's power, as will be demonstrated later. This is the path that the PostScript program is creating at the moment.