Dimensions establish the business context and represent the aspects of a client's business relevant to a rebate deal.
For almost every deal mechanism you select in DealTrack, you will be required to assign at least one item from each of your dimensions to the deal you are creating.
This setting is simply labelled Dimensions and will require you to first select a dimension collection and then to click on either a Select all items option or a Manually select items option.
Included dimensions determine what entities should earn rebate (e.g. which products or product groups or product categories are earning the rebate specified in that deal).