As with other public procurement procedures, changing the terms of the original tender in a Dynamic Purchasing System (DPS) requires a new procurement exercise, just as substantial modification to the terms of existing contract awards is not permissible without a new procurement procedure. However, in a DPS it is possible after tenderers have been admitted to the system, for contracting authorities to specify additional terms for the purposes of call-offs. Public Contracts Regulations 2015 (PCR 2015), SI 2015/102, reg 34 governs DPS procurement. Hence, the terms of PCR 2015, SI 2015/102, reg 34 must be the starting point for answering these questions. First, PCR 2015, SI 2015/102, reg 34(2) provides that all stages of procurement under a DPS must be by electronic means. It then provides that the DPS shall be open throughout the period of its validity