There are two types of tabs you can create.
The most common type is associated with the 2D Profile Toolpath and they are tabs created by the Profile Toolpath. They always start at the bottom of the toolpath and extend upwards by the Tab Thickness you have set. There are two different shapes that can be created, a flat topped version and a triangular shaped top. The triangular shaped type is called a "3D Tab" but it has NOTHING to do with a 3D component. It only describes the shape of the top and for most CNC machines they can machine a shape like this more smoothly and faster than the flat topped design.
If you are working with 3D models, like your .stl model, and want both the 3D Roughing and 3D Finish toolpaths to observe tabs, you need to create true 3D tabs, which are actual 3D shapes. The easiest way to do this in Aspire is to create rectangular shape, postion them where you need them around the model, and then use the Create Shape tool to make 3D models. If the Combine Mode is set to Merge, they will smoothly merge with the .stl model and neither the Roughing or Finish toolpaths will cut through them.
Hopefully this helps. If not, I can do a quick example for you.