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

EN Page Size Command 5-3
If the Page Size command selection differs from that of the installed
paper tray size and the requested page size is not currently available
in another non-secure source, then a message is displayed on the
control panel requesting installation of a paper tray of the specified
size.
Note The Page Size command is not supported on the HP LaserJet,,
LaserJet+, or the LaserJet 500+ printers.
Example
To select a legal size page, send:
? & l3A
If the current source paper tray is “LETTER” and the requested page
size is not currently available in another non-secure source the
following attendance message is displayed:
xx
1
‘‘LOAD LEGAL’’
1 xx = A two-letter mnemonic depending on the printer.
When the printer senses the existing paper supply has been removed
and replaced with the requested paper size, the print job continues
automatically.