what exactly dont you understand? keep in mind that kepware has quite a few settings that could override the ignition scan class setting. for instance, if you have kepware setup to request all data at 15000 ms, it wont matter if you have ignition set for 100ms scan class, because kepware will never attempt to scan any faster than every 15000 ms.
I think the common sense approach is to set your scan class to a reasonable number for the limitations of the equipment that you are using.