How are our buses numbered?

From GO Group Virtual Wiki

Bus numbers can be seen on the licenses plate after the letter combination.

8xx - Buses smaller than 10m
10xx - 10m buses
12xx - 12m buses (city)
13xx - 12m intercity buses
15xx - 15m buses
18xx - 18m buses
20xx - Buses longer than 18m (GL, AGG)
22xx - Double decker buses.

Right hand drive buses (UK, AU, JAP etc)

52xx - 12m buses (city)
62xx - Double decker buses.