This will be changing in the next update (It'll all be going to a singular @calc@ variable) but basically it's all in terms of how things are processed. @calcpre@ gets processed before an @if@ statement is called whereas @calcpost@ is done after the @if@ statement.
They both do the same thing other then this difference. As for how to use them...
@calcpre@: Executes before @if@ : @calcpre@[op,num1,num2] , e.g. @calcpre@[+,5,4]. Available operaions (+,-,*,/)
@calcpost@: Executes after @if@ :@calcpost@[op,num1,num2] , e.g. @calcpost@[+,5,4]. Available operaions (+,-,*,/)
Thanks,
Dante557