Which one do I use?
Well, when you're in the family editor you can draw symbolic lines, model lines or reference lines.
1. Symbolic lines only show in plan, elevation, or section depending on which view you're in when you draw them. Symbolic lines are typically used to "represent" door and window swings in plan and elevation.
2. Model lines will show in ALL views because they are part of the model. It's not very well documented but (while in the family editor) Detail Components placed perpendicular to a model line will "sweep" along the model line drawn (Search Augi Forums for Ultra Lightweight 3D Models to see an example). We also use model lines as a quick way to "represent detail" in some families.
3. Reference lines are essentially four intersecting reference planes. They can be used when you want to add a door swing angle parameter to your doors. Reference lines and planes should ALWAYS be used when using dimensioned parameters in a family. For more on reference lines and planes read Steve Stafford's Once Upon a Reference Plane.
When you're in the project you can draw detail lines or model lines.
1. Detail lines, like detail components, are view specific or part of the view. Anything you "draft" in a view will not show up in other views unless you copy and paste the elements into other views.
2. Model lines drawn in a project file are like those drawn in the family editor. They will show up in all views because they are part of the model.
To turn model lines into detail lines... select all model lines cut and paste into a filled region sketch. To turn detail lines into model lines... select all detail lines cut and paste into a floor sketch.