HP Color LaserJet 8550 Multifunction Printer series - Manual (Page 242)

242 Chapter 8 Administration Functions
EN
Inch Entry
The Inch Entry setting is used to switch the unit of measurement from
inches to millimeters. This setting will affect how you enter the
coordinates of a shift position, margin width, and other image shifting
settings.
To have millimeters as your default unit of measure the Inch Entry
setting must be turned off. To have inches as your default unit of
measurement the Inch Entry setting must be turned on.
Note The factory default setting for Inch Entry is on.