In order to identify a cavity and have the 3D visualization, first place a region where the cavity should be on a mechanical layer within the PCBLib. Then change the Kind attribute of the region on the mechanical layer to 'Cavity definition', and define the desired height of the cavity as it would be from layer to layer(if you want to completely expose the area or cavity you can set a very large height). You will see these options in the Properties panel if you select the region that is placed on the mechanical layer. This height defines the distance that the software will remove all layers above the surface of the copper layer that the component is placed on. You can use an additional mechanical layer for the cavity if you want to maintain separation for the callout layer for your board house. You can also add a 3D body to this component.To get more news about Cavity PCB, you can visit pcbmake official website.

Once the cavity has been defined for the component, you will essentially place the component on an inner layer(whatever layer the component should be on) and the orientation that the height will extend will depend on the orientation defined in the layer stack manager. Keep in mind only signal layers(positive layers) can have a component placed on them. So if you are using an Internal Plane(Negative layer) and wanted to place components on this layer, you would have to change that layer to a signal layer(then flood a copper pour on the layer to re-create the plane on the positive layer),
Next, within the Layer Stack Manager of a PCB, if you select a layer you will see an 'Orientation' in the Properties panel of the Layer Stack Manager. So if you placed the component on a signal layer 2 for example, if the orientation is set to Top, then the height will cut the copper towards the top layer and vice versa if the orientation is set to bottom.

To simplify the interaction between the cavity and the layers that it cuts through, the software ensures that a layer is not partially cut. If a cavity enters into a layer, such as a dielectric layer, but does not extend all the way through that dielectric layer, then the software automatically extends the cavity all the way through that layer.

You would still need to identify the desired location for your fab house, in addition to defining the cutout. You can also put an additional set of lines around the boundary of the cutout on a secondary mechanical layer to call out the area for your fab house.