Rack or Chassis
A chassis is the physical frame or structure of an automobile, an airplane, a desktop computer, or other multi-component device.
A Rack is the framework, typically with rails, bars, hooks, or pegs, for holding or storing things.
Q38RB-E - Mitsubishi
8 Slots, Base unit for 2x redundant power supply (Q64RP) + CPU + 8 I/O slots..
Q68RB - Mitsubishi
8 slots, extension unit for 2x redundant power supply (Q64RP) + 8 I/O slots..