In many situation, various operations of a plant or process take place in a particular order. A sequential control system involve the sequential execution of well- defined operations that are performed in a prescribed order. Each operations or activity is called step. Each step may be an open or closed loop continues process or even a sequential sub- process. For example, while using automatic camera, the various basic steps in a sequence are switch on, battery check, auto- focus the image, auto flash on/off, taking the image, saving the image and then switching off the camera.

Each step of the prescribed sequence usually requires a switching of the equipment configuration and it may be triggered by time or an event(push of a button, completion of early task etc.).