Have you read the manual on USAGE COMP-3 and USAGE DISPLAY? If not, start by reading the Language Reference on these first. If you have read the manual, since both types of variables are defined in there, we'll need a better sense of what you're asking.
BTW, they say - Packed-decimal (comp-3) has long been the preferred way to store things like quantities and amounts and for use in computations. Part of this was that comp-3 takes considerable less space than zoned-decimal. Another (and i believe larger) part is that the mainframe has assembler packed-decimal instructions that are required to "edit" values for readability. If an amount was stored in a comp field, internally it must be converted to comp-3 in order to show commas, a decimal point, a minus, etc.