It is with reference to the viewing angle.If the object is placed in the first quadrant (+ve X and +ve Y), and the plan is shown in the fourth quadrant, while the elevation is shown in the second quadrant, it is first angle projection.

If the object is placed in the third quadrant, the plan in the second, and the elevation in the fourth quadrant, it is third angle projection.

The firm and hidden lines differ because of the difference in the viewing direction. Third angle is easier to understand, because the plan is placed above the object, just as you would see from the top. The elevation is also just as one would view from the side it is placed at.

In 1st angle you have the 45 degree line that bisects the lines from the side view and front view in the bottom left. in 3rd view it’s on the bottom right and bisects the front view and the plan (top) view