Why do some London buses have codes painted on the top and some don't?

Question

Also what do they mean (workers in tall buildings and flats will know what I'm on about)

Answer 1

Some are part of the secret network and some are not.  now that you have let the secret out; how do you feel?

Answer 2

Buses should be gaining large fleet numbers and a code for the operator painted on the roof to aid identification on CCTV, not all operators have done this yet.

Answer 3

guess numbers on the top make them easy to spot from a helicopter

maybe some bus companies do it and others don't