Issue description
You have a slab on the plan and you would like to cut a hole in it to accommodate stairs, etc. When you use the slab tool to draw the shape of the hole, you end up creating additional slabs instead.
Solution
Let's go through the steps:
1. Select the existing slab.
2. Make sure your slab tool is Active.
3. Draw inside the boundary of the selected slab. This will now become a hole.
Tip: How to remove a hole from a slab.