Many of the codes are long-established, going back to London Buses Limited (LBL) (and in many cases earlier) days, and are well known among staff and enthusiasts. There is usually some sort of logic to the code allocated.
Several of these codes have an extensive past, tracing back to London Buses Limited (LBL) era (and in many cases prior to that). They are well-known among workers and hobbyists. Typically, there is some underlying logic behind the assigned code.