The bottom edge of the water bottle is located with respect to the lower water bottle boss. This measurement is taken parallel to the tube (shown here as 15mm). The side of the water bottle is located by an offset distance from the edge of the tube (shown here as 14mm).
The water bottle bosses are located by a distance from the bottom bracket. BikeCAD Pro calculates the location of the water bottle boss from the end of the tube. This is illustrated below.
BikeCAD permits up to three water bottles. You can turn them on and off with the numbered check boxes in the Water bottles section of the Accessories tab. Bottle 1 is located on the top of the down tube. Bottle 2 is located on the front of the seat tube. Bottle 3 is located on the underside of the down tube.