Some transit agencies use a 5 digit numbering scheme despite not having over 10000 buses. Under the most common 5 digit scheme, the first two digits represent the year the bus was purchased, the third digit represents the size of the bus (3 for 30-35ft, 4 for 40-45ft, 6 for 60ft), and the last two digits represent the fleet number within that series. For example, 20317 would correspond to the seventeeth 35ft bus purchased in 2020. In this picture, the bus is a 35 foot Gillig Low Floor bus. The numbering makes sense, but the MBTA has never owned a Gillig bus (their primary vendor is New Flyer) and has not had 35 foot buses since 2007. The MBTA also uses a 4 digit numbering scheme that does not incorporate purchase date or bus size.