This variant is designed for use with 8 HP-wide power supply units.
8 HP-wide - ?

Еще подобное место:
1 Slot = 4 HP = 20.32 mm

Кстати, тот же текст имеется на немецком, там вместо HP - TE.

http://www.pentek.com/deliver/TechDoc.cfm/Techtalk.pdf?Filename=Techtalk.pdf
What is HP?
HP stands for Horizontal Pitch and is equal to .2 inches. Usually in reference to width, this unit of length is commonly used to size VMEbus, CompactPCI, and Advanced TCA enclosures and backplanes.
For example, the width of a VMEbus and CompactPCI board is 4 HP
or .8 inches and AdvancedTCA is 6 HP or 1.2 inches.