20's are double decks
10's are single decks
Internally vehicles in the ECB fleet are referred to with their full number e.g 20950 and not by the shortened, wider fleet numbering system of 950 - as many enthusiasts would have you believe.
I will say I’ve been into Musselburgh garage a few times (with permission obviously, I was speaking to Mark Heritage about jobs - I ended up getting a job in the railway) and the drivers there picking up the duty cards always referred to them by the short number (this was prior to the B5TL’s arriving so it must have been in 2017 when ECB was fairly new). Perhaps since then this has changed, I imagine on the radio they’ve always been referred to by the 5 digits though.
I suppose for two reasons I don’t use the full number, if I’m talking to a friend about ECB chances are they’ll still know what I’m talking about, I don’t work for them. Secondly I don’t work for Lothian (mainly as I’ve been turned down too many times) so as a member of the public I’ll stick with what I know if that make sense, ultimately I know it’s fleet number is 20950 and yes I’m wrong to think it’s anything else but I’ve known it 950 since it was knew. TfE app also can only show up to four digits, another thing that makes me not use the full number.
I don’t wanna get too off topic but in my work we don’t refer to Headcodes fully all the time (only drivers and train managers do, signallers we have here are even worse and often don’t use phonetics) unless an important message is passed out on the radio - usually set swaps.