In a power system, electricity generation and electricity consumption must always balance. Changes in consumption and disturbances at production facilities impact the system balance and cause frequency deviations in the grid.

The need for ancillary services (including regulating power reserves) is dynamic throughout the year, both in terms of volume and the nature of the ancillary services, just as there are regional differences between Eastern and Western Denmark. Consequently, the volumes and services on offer are adapted to requirements for specific periods of the year in Eastern and Western Denmark.

The ancillary services, which are bought from electricity producers and electricity consumers in Denmark and its neighbouring countries, are used for different purposes, and therefore the requirements for how the services are to be supplied differ as well. These requirements are regulated in the ENTSO-E CE Operation Handbook - Policy 1, the Nordic System Operation Agreement and's regulations for grid connection.