A system unit is a standard modular backplane from DEC, used mostly in UNIBUS PDP-11s.
The initial system units were hex height, four-slot assemblies. (Sometimes a number of these were wired together to form a single unit, such as in the PDP-11/20's KA11 CPU, which used three.)
Late on, 'double' system units (also hex-height, but nine slots wide) appeared. There were also custom backplanes (such as for the PDP-11/45 and PDP-11/70, the latter for both the CPU and memories such as the MJ11 and MK11) which were even wider than that, but generally a double was the maximum width for an option.
All the DEC chassis boxes (such as the BA11-K and BA11-L) had mechanical mountings and DC power connectors intended to hold system units.