If I remember correctly, last time, the first digit of the rego
indicates which depot the bus belong to. Apparently, sbs 1-999 are
reserved for aircon buses. They are regarded as a special service
back then so they deserve special regos. As for the B9TL, I can see
a bit of problem here. There are not enough regos to go if they
start from either SBS 9691 or SBS 9850. I guess the only way is to
split them; unless SBS has other ideas.