piaggio 125 4v engine

[WDJ]

hello, everyone
i query parts on the www.easyparts.nl
i find the same piston have 1/2/3/4 cat and the same crankshaft have 1/2 cat, what is the meaning and different?

http://www.easyparts.nl/showdrawing...m6260-m2231-s2184-d39446/Cylinder_zuiger.html