From the introduction of TOPS in 1973, all British Rail diesel and electric locomotives and multiple units were allocated to a particular traction maintenance depot or TMD.
Locations given a TOPS code included all TMDs, servicing depots (where minor work such as A exams could be undertaken), and some stabling points (SPs) where locomotives or multiple units would be parked when not required.
The locations varied from comprehensively equipped depots, through covered accommodation with fuelling point, to a mere siding.
As there was no fuelling or maintenance capability at BD, a shunter allocated to nearby Birkenhead Mollington Street (BC) was used, returning to its home depot every four weeks.
Departmental locomotives, by the nature of their use, were sometimes reported as allocated to a base location or even to a region in general rather than an official depot.