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It's implicit in the timetables. Took me a while to figure it, too, and I'll try to explain it as best as I can.
If you look at the beginning of the timetable document, for each bus line it states all the "sublines" and where they go, and notice that each subline has a different final destination or path than the others subline.
Let's take as an example the 6C line, which start from San Colombano and stop at Stezzano Est. It's not listed explicitly in the timetable (welcome to Italy), but check the timetable row for Stezzano Est and notice that only a few columns have the time of arrival for it and the leaving time from San Colombano: those are your 6C buses. For example, on the summer timetables, the 7.12 from San Colombano is a 6C.