I don't think the paragraph explaining the :PRINTER arg (Printer s/w
guide p. 4) is clear.  What you may want is something like a section on
the key variables, and their format, to introduce basics like a printer
specification; then, a breakout on the hardcopy :PRINTER arg.

Here's some stuff I threw together:

...The argument is either:

1) a list containing a printer specification
2) a string representing a printer nickname
3) a symbol or expression which evaluates to a printer specification or
nickname
4) a PRINTER KEYWORD which is referenced in tiger:*default-printer-host-alist* 

A printer specification is generally a list whose CAR is a keyword
symbol specifying the printer type (e.g. :TI855, :LASER1) and whose CADR
is a string containing the name of the host where the printer resides...

Your reference on printer nicknames is fine.

Some examples here might be helpful.  For example, various equivalent
ways of printing a file to a :TI855 on the host "LAMA" would be:

1] Spell it all out:

   (HARDCOPY-FILE "MYFILE" :PRINTER '(:TI855 "LAMA"))

2] Given 

   SI:*PRINTER-NAMES*
      => ((("TI") (:TI855 "LAMA"))
          (("HP" "QUICKY") (:LASER1 "LAMD")))

Then:

   (HARDCOPY-FILE "MYFILE" :PRINTER "TI")

3] Either

   (SETQ FOO  '(:TI855 "LAMA"))
   (HARDCOPY-FILE "MYFILE" :PRINTER FOO)

or
   (SETQ FOO "TI")
   (HARDCOP-FILE "MYFILE" :PRINTER FOO)


4] Given 

   TIGER:*DEFAULT-PRINTER-HOST-ALIST
     => ( (:TI855 "LAMA") (:LASER1 "LAMD"))

Then:

   (HARDCOPY-FILE "MYFILE" :PRINTER :TI855)