The PinShownByDefault specifier is used exclusively by a handful of animation nodes. Properties of an animation node can be exposed either as pins on the node or as fields in the Details panel. This specifier ensures that a given property appears as a pin on the node, and it may also be viewed as a field in the Details panel. The UI will contain an eye icon that can be used to toggle visibility of the property pin.
It must be combined with BlueprintReadWrite and EditAnywhere.