A CompactPCI system is a 3U

Rack unit

A rack unit or U is a unit of measure used to describe the height of equipment intended for mounting in a 19-inch rack or a 23-inch rack...

 or 6U Eurocard-based industrial computer, where all boards are connected via a passive PCI

Peripheral Component Interconnect

Conventional PCI is a computer bus for attaching hardware devices in a computer...

 backplane. The connector pin assignments are standardized by the PICMG US and PICMG Europe organizations. PICMG

PICMG

The PCI Industrial Computer Manufacturers Group is a consortium of over 227 companies. The group, founded in 1994, was originally formed to adapt PCI technology for use in high-performance telecommunications, military and industrial computing applications but its work has now grown to include...

stands for PCI Industrial Computers Manufacturers Group. The connectors and the electrical rules allow for eight boards in a PCI segment.