When creating a family, one can indicate where the family origin is.
One must indicate that a reference plane defines the origin. Question is, which reference planes qualify?
Reference planes may be drawn at any angle, but only vertical and horizontal reference planes accept the notion of being origin defining. Yes, one may draw a slanted reference plane and then set it to define the origin, but this really has no meaning in Revit.
Also note, if there are more than one vertical or more than one horizontal plane, one of the planes in each direction accepts the property. For example, two vertical planes, A and B, exist. A is currently assigned the origin. Now the user assigns the origin to B. Revit automatically revokes the origin from A and assigns it to B.
If you need help adopting BIM or using Revit, please contact Micrographics so we may be of assistance.