In addition to 2D and 3D compositing, HitFilm allows you to overlap or combine the two using depth maps. Working with models or effects in 3D Unrolled makes it very easy to arrange them interactively within HitFilm's Unified 3D workspace. But working in 2D provides the advantage of being able to apply effects directly to models or particle systems. Using depth maps, your layers can remain in 2D, so effects can easily be applied, but they can still be rendered accurately into the depth of the 3D scene.
Depth maps are greyscale images that represent the distance from the camera to each object in the scene. Once generated, the depth map can be used as a matte, and applied to to other layers to control their visibility.
When a 3D model or particle simulator layer is set to 2D, the Layer Properties will include a Depth Source Layer menu. You can use this menu to select any other 2D layer on the timeline, so that the layers will be rendered as if they were 3D, using the depth map.