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From Knowledge Article Number000206759 - Spring '15 - Invocable Actions with APIs. (https://help.salesforce.com/HTViewSolution?id=000206759&language=en_US ) (my emphais)
All invocable actions are invoked using a common request object so you don’t have to interface with different SOAP objects or REST JSON body shapes. Invocable actions provide “describe” support so you can programmatically learn what each action’s valid input parameters are, as well as an action’s output values.
Invocable actions are available only through the REST API and provide the following attributes:• Dynamic input and output values• Programmatic describe support• Consistent invocation interfaceThe following actions are available:• Standard actions—These actions can be invoked as-is and don’t need any setup or creation of an instance.– Email– Post to Chatter– Submit for Approval• Custom actions—You create these actions. Apex actions are typically created by a developer, while the other actions are created by an administrator.– Apex– Email Alert– Flow– Quick ActionFor more information on invocable actions, see Invocable Actions in the Force.com REST API Developer’s Guide (https://www.salesforce.com/us/developer/docs/api_rest/).
Note that I had to extract that content from the Google Cache. For some reason it wasn't appearing on the page.
