If I recall correctly, the second subunit box is unused. I don't think that anything actually puts things in there. I may be wrong, but I'm pretty sure that you can't have two subunits. I just don't think it's possible. If I am wrong, someone please correct me.
Yeah, well, it is called Subunit 2 because this property is located in the DAT right after Subunit1. No unit seems to use it though, so I think calling it Subunit2 is misleading.
DoA>Can you check if this property is ever used at all? If not, then it'll be marked as unused for the next version of DatEd.