diff --git a/gdtf-spec.md b/gdtf-spec.md
index a540ffd..6ed3b3b 100644
--- a/gdtf-spec.md
+++ b/gdtf-spec.md
@@ -512,6 +512,7 @@ currently defined XML attributes of the wheel slot are specified in
 | Color              | [ColorCIE](#user-content-attrtype-colorcie )   | Color of the wheel slot, Default value: {0.3127, 0.3290, 100.0 } (white) For Y give relative value compared to overall output defined in property Luminous Flux of related Beam Geometry (transmissive case).                                                                                                                     |
 | Filter             | [Node](#user-content-attrtype-node )       | Optional. Link to filter in the physical description; Do not define color if filter is used; Starting point: Filter Collect                                                                                                                                                                                                       |
 | MediaFileName | [Resource](#user-content-attrtype-resource ) | Optional. PNG file name without extension containing image for specific gobos etc. <br />- Maximum resolution of picture: 1024x1024<br />- Recommended resolution of gobo: 256x256<br />- Recommended resolution of animation wheel: 256x256<br />These resource files are located in a folder called `./wheels` in the zip archive. Default value: empty. |
+| SpecialFunction    | [Enum](#user-content-attrtype-enum )   | Defines special function of the Slot. Currently defined values are "None", "Open" - the slot is not affecting the beam, "Closed" - the slot is completely blocking the beam. Default value is "None". |
 </div>
 
 As children the attribute node can have a list of [Prism