HP PCL 5 Reference Manual page 275

Notes
This command is used for both the Select Pattern and Area Fill
graphics (it is also described in Chapter 14, PCL Rectangular
Area Fill Graphics). It is duplicated here for convenience.
For user-defined patterns, this command, sent prior to downloading
a user-defined pattern, assigns an ID pattern number to the
downloaded pattern. (For more information, see "User- Defined
Graphics," later in this chapter.)
EN
Pattern ID (Area Fill ID) Command 13-9