Say you’re making a window sash to put into your custom window family. You open up the generic family template, but you’re not sure what to do next. How should you orient your geometry considering you’re going to load it into another family?
Geometry in the generic family template is hosted on the reference level, so if you build it parallel to that plane and then try to load it into another family and rotate it to be perpendicular, you’ll get an error message. So you should always consider the final orientation of your geometry relative to the reference level and start it out right.