A ridgeline marks the apex of each of the three roof sections on this roof plan.
Drawing a floor or a roof on a different plane.
In the two examples shown below the wall plans are identical apart from the parapet wall in roof b yet the two roof shapes are quite different as indicated by the roof lines on the plan drawings.
As with designing a floor plan layout the roof must also be considered when establishing the footprint of the house.
Keep it simple stupid.
The plan is typically cut at a height of about 4 feet but the architect drawing the plan may cut it at a different height.
For each offset angle or change in plate.
This means that you have an imaginary plane cutting through the building at an elevation of 4 feet above the floor.