If a vendor offers a product or group of products with special costs, perhaps for a limited time, instead of defining a new REP-COST on the product file for those items, you would create a new buy matrix cell with the extra discount that is in effect for the specified period of time.
When that matrix cell expires, the system reverts to using the normal REP-COST (REP-COST *1) for the product or group of products. This cell might also have expiration quantities that mean when the specified number of items has been purchased, the system will revert to using the normal REP-COST.