Model lines and detail lines may seem very similar to the lay person. They both contain the same tools, line types and look the same in the view you drew. They are available in both projects and families. So what is the difference?
The fundamental difference is that Model Lines show up in every view, and Detail Lines show up only in views perpendicular to the plane they were drawn. So if you want to draw some linework to show up in an elevation view, but don't want to see it in plan or in 3D, then use drafting lines.
Here is an example recently run across: Lines were drawn on a mezzanine floor plan to indicate the position of permanent casework in the open area below. The lines were drawn with model lines which meant when elevations were taken of the wall, a single line showed up at the elevation which obviously wasn't ok. All the lines that were previously model lines had to be converted to detail lines, which luckily was easy enough as clicking a button.
On the other hand, in a door family, the 'elevation swing' lines are usually drawn as detail lines, so they show up in front and back elevation views only. We had a situation where the wall being elevated was at 3 degrees from the cut plane of the elevation, so naturally the elevation swing lines didn't show up because it wasn't perpendicular. To fix this issue a set of 'elevation swing' lines were drawn as model lines in the door family, then when it was re-imported into the project they showed up as they should on the elevation.
