HP Color LaserJet 5500 Technical Reference Manual page 249

Note
Printer Environment
Under these conditions, the printer samples the incoming
printable data and looks for indications of a particular
printer language. Once it recognizes the printer language,
the printer backs up to the beginning of the sampled print
data and switches to the printer language determined to be
most appropriate. Then the printer begins to parse the data
in the newly selected printer language.
Although the printer can accurately select printer
languages this way, we recommend that every job
containing printable data include an ENTER LANGUAGE
command to explicitly switch to the correct printer
language. This method improves performance and
eliminates errors in printer language switching. It also
guarantees that the correct language is always selected.
The value of the PARALLEL feature as shipped from the
factory is FAST. When a front panel reset is performed, the
value is defaulted to SLOW.
Printers that support printer environment variables:
HP DeskJet 1600C, 1600M
Unlike most PJL variables, the following environment
variable defaults are not stored in the printer's NVRAM.
The primary use is to set job-specific parameters like those
available through the DeskJet 1200C family front panel.
This means that they will be reset to the factory default
values after a power cycle.
Feature Support A-37