How to print source code from the command line

[Alex Kleider]

On 2015-01-01 17:25, Ken Shaffer wrote:
> My favorite a2ps output is:
> a2ps -C -MLetterdj -2
> which gives me a tiny line number every 5 lines, and is 2 up,
> (on a deskjet device, change for your printer).  All the other headers,
> footers, and boarders are standard.
> Ken

excerpt from
man a2ps =>
...
       -M, --medium=NAME
               use output medium NAME
...
        -1, -2, ..., -9
               predefined font sizes and layouts for 1.. 9 virtuals
...

Where and how are the "1.. 9 virtuals" "predefined"?
..and what is the output medium?  ?specifies a printer?

Can you shed some light, please?

thks,
Alex