In most Member States, centralised purchasing techniques are increasingly used. Central purchasing bodies are responsible for procurement, the management of dynamic purchasing systems or the award of public contracts/framework contracts to other contracting authorities, with or without remuneration. Contracting authorities for which a framework agreement is concluded should be able to use it for individual or repetitive purchases. Given the large quantities purchased, these techniques can contribute to increased competition and contribute to the professionalisation of public procurement. It is therefore appropriate to provide for a Union definition of central purchasing entities for contracting authorities and to clarify that central purchasing bodies operate in two different ways. When awarding such contracts, contracting authorities may consult in writing the economic operator which is a party to the framework agreement and invite him to complete his tender if necessary. where the changes, irrespecty of their monetary value, are provided for in the initial procurement documents in clear, precise and clear verification clauses, which may include price variation clauses or options. These clauses specify the extent and nature of possible modifications or options, as well as the conditions under which they may be used. The possibilities provided for in paragraph 1 of this paragraph shall also apply to all consignments of a framework agreement for which all the conditions for the execution of the works, services and supplies concerned are laid down in the framework agreement, whether all the conditions for the provision of the works, services and supplies concerned have been laid down in the framework of other lots. .