Fault lines define the area where the shooter is allowed to be while shooting.
You can build them using the same principle as walls.

Optionally you can plan a route and shooting positions. This will allow you to make sure the target will be in a line of sight and to play the simulation inside the Stage Builder.

Select the “Path for target” tool from the panel on the left and build the approximate route. Also place the points in the places where you’re planning to shoot corresponding targets from.
Then select the first point from where you’re planning to shoot, click and hold the “Set target” button and drag it onto the desired target. A 3D model of the shooter and a trajectory of firing will appear. You can fire multiple targets from one route point, just repeat the same procedure for a different target.

If you select the shooting trajectory with left click, the distance to the target will appear.
If the shooting trajectory is red, it means the target will not be in a line of sight from this position. It can be fixed by moving the route point, or the target itself.
If you’re firing through a hole in a wall, or you need to peek to fire at a target, you need to adjust the shooting position. Click and hold buttons with up and down arrows, drag them for adjustment the height and peek.
Do it for all targets.
If the route goes through the Fault line, it will result in an error.
If you made everything correctly, when you press the “Start” button in the bottom left corner, the shooting simulation will start.