HP 4 FAMILY - Manual (Page 169)

All HP LaserJet 4, 5, and 6 family printers also include
enhancements to improve the internal storage of raster
graphics data.
Additional MEt
Features
With MEt, font data can also be compressed. All
downloaded bitmap characters and characters scaled from
internal or downloaded scalable outlines can be
compressed. The amount of memory savings varies with the
size of the characters involved (larger is better) but
character sizes can typically be cut in half. This allows
roughly twice as many fonts to be downloaded or scaled
using a MEt-enhanced LaserJet printer as previously
allowed on printers without MEt.
In prior PCL 5 printers, if the page could not be rendered as
fast as the laser printed it, a Print Overrun error occurred
unless page protection was manually turned on and
memory was added to accommodate it. With MEt, the
manual setting of Page Protection and the additional
memory required to facilitate it is obsolete. MEt
automatically assesses the complexity of the page being
printed and if too complex turns on a new form of page
protection which uses compression to remove the
requirement for additional memory.
In rare circumstances MEt’s assessment of the page may
prove to be incorrect. For these cases there is some amount
of control over this Page Protection process depending upon
which MEt-equipped printer is being used. Page Protection
may be explicitly turned on or off to avoid the complexity
assessment and either always or never perform the page
protection process. See the appropriate user manual for
details.
MEt also includes other internal memory-saving techniques
which improve the amount of memory required for the
printer’s internal representation of your page.
Memory Usage 5-3