Returns the value of ThresholdDepth attribute. This attribute represents depth (dimension in plane perpendicular to door leaf) of the door threshold. Only given if the door lining includes a threshold. If omitted (and with a given value to threshold thickness) it indicates an adjustable depth (i.e. a depth that adjusts to the thickness of the wall into which the occurrence of this door style is inserted).

Returns the value of ThresholdDepth attribute.