Many documents don't have a clear enough of a visual dinstinction between the two.

At the very least, you should use a proportional font for prose and a monospace font for code. However, IMHO, this isn't always a clear enough distinction, particularly when the operator already understands the procedure. In this case, they want to completely ignore the human-reaedable text (because they understand it already) and quickly scan for only the computer-readable text. (because it's slightly more difficult to remember every character exactly)

On screen

On-screen, I prefer to use a distinct color, either the foreground or background, to signify text that's intended for the computer.

Black-and-white printouts

However, in my experience, many black-and-white printers vary their gray levels quite a bit over the life of the toner cartridge, getting lighter as toner level falls. This makes it difficult to use slight-gray-backgrounds, because the gray level needs to be very preceise (not too light or it becomes harder to distinguish from white-background text, and not too dark or it becomes harder to read the black-on-dark-gray text).

If your printer is among the majority that have uneven gray levels, it's better to do this:

• Reserve bold styling for computer text ONLY.
• For section headings, rather than using bold letters, instead use reverse color lettering (white foreground, black background).
• For all places that you would otherwise use bold within prose, substitute underline or italics instead.

Color print-outs