HP PCL/PJL reference (PCL 5 Printer Language) - Technical Reference Manual Part II

24-2 Programming Hints EN
PCL Command Parsing
A job stream may contain commands that are device specific.
If these commands are not supported by the PCL device, they are
ignored. For example, a Duplex Print command has no effect on
the HP LaserJet III, IIIP or LaserJet 4 printers, since these are
non-duplexing printers. However, on printers which support duplex
printing (such as the HP LaserJet IIID and LaserJet IIISi), the job
is printed in duplex mode.