Shadow masks are transparent shadows that can save you a lot of time during baking process.
Here is how to use shadow masks:
1. First make a proxy mesh for the shadow receiving area.
2. Make sure the mesh has proper unwrapped UVs.
3. Assign a Material that represents the shadow color to the proxy mesh.
4. Select the proxy mesh and from the X-Ray Window click on the Mask button 
5. You can now bake the proxy mesh.6. To make sure the result is viewed correctly you can press the Update Shader button 
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7. After that, you can change the intensity and coverage of the shadow area by clicking on SRamp button 
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