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

Glossary-16 EN
Vertical Motion Index (VMI)
VMI (vertical motion index)
VMI defines the distance between rows in 1/48th inch increments.
This command affects the Line Feed and Half-Line Feed spacing.
The factory default VMI is eight, which corresponds to six lines per
inch. A user default VMI can be selected from the control panel using
the FORM menu item.