Definition of "subunit"

The bogies are built up of welded sub-units which are stress-relieved before assembly by riveting.

1961 October, “New Metropolitan Line train sets enter service”, in Trains Illustrated, page 622

Interchange manipulation comparisons showed that all subunits are still active, if at slightly lower rates.

2007 September 25, Bungie, Halo 3, v1.0, Microsoft Game Studios, Xbox 360, level/area: Terminal Three (Legendary)