I understand most all of the codes but this one:

It has for:

119" BBC Conv Cab Tractor 6X4
119" BBC Conv Cab Tractor 6X2
121" BBC Conv Cab Tractor 6X2
121" BBC Conv Cab Tractor 6X4

What does this mean, and which one would I pick for a Day Cab?

Papa see you found the blackbook.
I think the BBC stands for the distance from the front bumper to the back of the cab. like the long nose pete and the short. the 6x4 is for tadem axle and the 6x2 is for single axle. There should be a glossry somewhere to help. If I remember correctly you select the options like day cab or sleeper.