An activity is a process, i.e. the combination of actions that result in a certain set of products. Activities are defined as the use of inputs (e.g., capital, labour, energy and materials) to produce outputs. The outputs that result from undertaking activities can be transferred or sold to other units (in market or non-market transactions), placed in inventory or used by the producing units for own final use. In practice the majority of units carry on activities of a mixed character. One can distinguish between three types of economic activity: Principal activity: The principal activity is the activity which contributes most to the total value added of the unit under consideration. Secondary activity: A secondary activity is any other activity of the unit that produces goods or services. Ancillary activity: Any ancillary activities are those that exist solely to support the main productive activities of a unit by providing non-durable goods or services for the use of that entity.