hello lex,
C2 can be used for reselection, it is an option that is activated through another parameter.
In this case, the MS reselects the cell with highest C2, as long as this cell also verifies C1 > 0.
The only difference is that the C2 is using an offset that can be fine-tuned by the optimizer (the cell reselct offset), as to favour specific bands or layers in the network.
multi-RAT = multi radio access technology = 2G + 3G + 4G