The attribute data type of 'action' represents a string that can be used as an action; under the hood, Tinderbox caches the compiled action so it does not have to constantly re-parse the same expression. Thus these attributes should contain a string [sic] of valid Tinderbox action code.
Some pre-existing system attributes, such as OnAdd and Rule have been re-classed from 'string' to 'action' type.
This data type is not available for user attributes. Use 'string' instead.
An empty string.
As for String Data Type.
Action-type System Attributes
Built-in attributes of the action data type are listed below: