The selection also works in the plug-in mode. You can import into PostworkShop a part of your PSD layer: simply create the selection in PS, and invoke the plug-in.

Load the image you want to apply some styles to. You could also use a PSD file, with more layers. In this case select the layer you want to apply the styles to. To see first the effect we will be applying and to work more quickly, we will add this style on a portion of the image, we will therefore create a selection. If you are not sure about the fact that you will like the result, it is recommended to duplicate the layer you are working on in Photoshop. 

Once selected the portion of the background image we want, call the PostworkShop plug-in from the Filters/Xycod menu.Only the selection will be loaded in PostworkShop and the rest of the PS layer will be a transparent checkerboard since the Canvas is set to Transparent in the Preferences menu. If the canvas had been an image or another color, you would have seen that image or that color behind the image portion imported from PS. Just like for any other image, an Object is created with its default Layer #1. 

Apply styles, modify and blend them if needed. We applied only the Dawn style from the Photo category. 

When you are done press the Apply or Apply&Exit button. We clicked the "Run the final render and return to the host" button. The image in rendered and progress bar appears.

Then the selection image is sent back to Photoshop.

In the History window, you can see PostworkShop, as a change made to the image. Now you may want to apply that same effect to the whole image or at least, to the remaining part. Do not unselect the selection, but right-click and choose Invert Selection. Now from the Filter menu, click the PostworkShop filter on the top. this will recall the last used PostworkShop styles.

When you select to recall the last used PostworkShop filter, PostworkShop will automatically go through the same steps as you did before: it will open up, the image -part- will be loaded and the style you applied before will be added to the image. Then it will go back to PS. As you can see below, now all the image has been added the Dawn style. 
Deselect to see the final result.