The H960 rack and H961 rack (officially the H960 cabinet and H961 cabinet), built around the H950 frame, were DEC's standard 19" wide rack (an industry-wide standard width for rack-mounted gear), used with the PDP-8, PDP-11, etc.
The H950 is the bare rack; the H960 included add-ons such as the rear door, side panels, etc. The H961 was intended mostly for expansions, and did not include the side panels, but did include a pair of filler strips (to go between the racks).
It was 6' tall, and provided 63" of mounting space; DEC also came out with some shorter 19" racks (e.g. the H967 rack). The mounting space in the H960 was divided into 6 10-1/2" units, each of which could be further sub-divided into two 5-1/4" spaces. Most DEC gear was designed to fit in spaces of these sizes.