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The Smart Filters thumbnail in the Layers Panel is a **mask**. Treat it like any other mask and it will hide/reveal areas of the filter.

[![enter image description here][1]][1]

- Add the smart Filter, 
- Fill the Smart Filter thumbnail with black
- Make your selection
- Fill that area on the Smart Filter with white.

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This does *not* assist with distortion filters such as Spherize. You will need to separate pieces to apply the spherize filter to a specific area. You simply can't use those type of filters on *part* of a layer.

You can *mask* a protion of the filter, but the result isn't quite the same:

[![enter image description here][2]][2]

Much like CMYK/RGB and filters.. some things only work one way if the image itself meets requirements. Smart object are seen as a whole layer for smart filters.... you can only mask the filter, not restrict it to a specific area. Raster content is required if you wish to restrict a filter to a selection.


  [1]: https://i.sstatic.net/dXEHX.png
  [2]: https://i.sstatic.net/9WSkM.png