Other sub-actions occur in parallel, such as, for example, the Stir action which in XDL is implied for many other steps, such as Dissolve, Add, HeatChill, and many more. Active heating or chilling may accompany a Dissolve or other reaction action. In the SPL processStep model, parallel activities can be specified as component activities. The difference between sequential and parallel steps is indicated simply by a sequence number. Those processStep components with sequenceNumber 1, 2, 3, 4, … are sequential steps, when more than one processStep component has the same sequenceNumber, they are occurring in parallel.