But they mention the Century Class S/T, which is indeed listed as different from Century Class, but that does not help much for the difference between Century and Century Class, if there is one.

I have only found one discussion about it: http://www.ask.com/answers/91172001/what-is-the-difference-between-freightliner-century-and-century-class-st

The limits of WP contributors are visible when it is about medium- or heavy-duty vehicles. I searched first in the WP, of course, and just hoped that someone of the IMCDb crew had some more infos about these trucks... But let's use WP as source. For now

Wikipedia does not mention a Century alone, but either Class or C120
http://en.wikipedia.org/wiki/Freightliner_Trucks#Models
so I'd say we could rename Century to Century Class, but keep the S/T and C120 as they are since they are listed that way on Wikipedia.

What are the (or is there) differences between Freightliner Century and Century Class trucks? Shouldn't these 2 categories be merged?

What about Century Class S/T models or Century C120 ones? Are they "sub-models", separate ones or newer versions of the older models?
Freightliner apparently sells only the Century Class S/T by now, in USA as well as in Australia: is it the new name?