Used to indicate the availability of an interactive service.

A trigger is used to provide a prompt to announce an interactive service and offer a call to action to invite the user to enter an interactive application.

In the UK the term Red Key Trigger is used to describe a prompt that invites the user to press the red button on their remote control. This is conventionally used to launch an interactive application.

On the BSkyB OpenTV satellite platform the presentation of the prompt is controlled by a specific data component often referred to as a trigger track. This carries a small loader application that includes icon graphics to present the default and loading state. It also provides a reference to a destination data component and an optional context string. These are used to target the trigger at a particular application and specify a launch state. The presentation of the trigger icon is controlled by the EPG application in the set-top box.

On the Freeview terrestrial platform the presentation of a trigger icon can be simulated in an MHEG application.

In the US ATVEF system triggers are carried on a closed caption channel in line 21 of the NTSC signal.