This follows on from Strapless.
Contouring marks indicate where gaping will occur. Gaping means that a dart is required, so the solution is to put in a dart.
In the example in Figure 1, the design line crosses one of the gape dart lines, so a dart is required. This does not mean putting a dart in the neckline** - the gape dart needs to be incorporated into the design. In this case, it will be moved into the side seam dart.
Moving darts to difference places in the Bodice Block is covered in detail in the Manipulating Darts section of the Principles pages, and is a skill you need to learn before attempting to make your own patterns. Below in an example of how this gape dart information is applied; in this example you can see the difference in the shape of the pattern piece when the gape dart is moved to the side seam dart.
** Note: You can put a dart in the neckline if the design calls for it, but in this case it doesn't.
The Gape Dart Moved
In Figures 2 & 3 below, the gape dart has been moved into the side seam dart.