|| This linedef type does not work. Although the feature is recognized by SRB2 and listed in map editor configurations, it is either disabled or broken in the current version of SRB2.
Linedef type 21, PolyObject - Explicitly Include Line, is a type of linedef special which is currently broken in SRB2 2.1. It was intended to be used on linedefs to explicitly include them in a PolyObject instead of using the First Line special, with the linedef's tag set to the tag of the PolyObject's Parameters. A parent PolyObject is also required for the PolyObject to appear in-game. However, the resulting PolyObject will always be invisible and intangible in-level, and is otherwise useless for SRB2 maps.