Ancillary services are services that are essential to the management of power system security, facilitate orderly trading in electricity and ensure that electricity supplies are of acceptable quality. Market ancillary services are ancillary services which are acquired by AEMO as part of the spot market in accordance with this Chapter 3. The prices for market ancillary services are determined using the dispatch algorithm. Non-market ancillary services are ancillary services which are not acquired by AEMO as part of the spot market, but under agreements which are entered into following a call for offers in accordance with this rule 3.11. The prices for non-market ancillary services are determined in accordance with the relevant ancillary services agreements.