Not all back-up are QBOTF. The likes of Trubisky, Wentz and Flaco are not QBOTFs but they are backups, most but not all QBOTF are back-ups, it is quite likely thart Tyrod Tailor is the Jets back-up next season but Jordan Travis also makes the cut as 3rd string QB and QBOTF.
I would define a...