Head tube bottom is a common dimension used with framebuilding fixtures to locate the bottom of the head tube relative to the bottom bracket. A few fixtures have their own variation on this dimension and BikeCAD has the option to display them all.
However, there are some fixtures that aren't configured to measure directly to the bottom of the head tube. Instead, they'll have a cap or some form of insert that connects to the head tube requiring that head tube bottom measurements are taken at some offset from the bottom of the head tube. BikeCAD version 13 allows us to account for these offset measurements with Dimensions X and Y in the Head tube bottom extension tab of the fixturing dialog box.
Even though the X and Y Head tube bottom extension dimensions are solely dependant on the fixture being used to build the bike and are not properties of the bicycle frame itself, they are nonetheless stored in the BCAD file along with the other dimensions defining the frame geometry for a particular bicycle design.