An actionable attribute is a model attribute with a modifier of "KnownBy" pointing to one or multiple Action(s). For example:
<String name="Image" KnownBy="SyncImage"/>
<String name="State" KnownBy="SyncImage,SyncPreset"/>
An actionable attribute defines the active property of a ThingFacets or AppFacets.
In runtime, an actionable attribute will activate the associated Action(s) and their workflows whenever its value is modified.
For more on the relationship between Actions and actionable attributes, see here.