(66)In order to further the possibilities of SMEs to participate in a large-scale dynamic purchasing system, for instance one that is operated by a central purchasing body, the contracting authority concerned should be able to articulate the system in objectively defined categories of products, works or services. Such categories should be defined by reference to objective factors which might for instance include the maximum allowable size of specific contracts to be awarded within the category concerned or a specific geographic area in which specific contracts are to be performed. Where a dynamic purchasing system is divided into categories, the contracting authority should apply selection criteria that are proportionate to the characteristics of the category concerned.