Quotes from the Lightroom Help File- Adobe Photoshop Lightroom Help | Lightroom Help
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Underlined sentences refer specifically to your question.
Apply a Radial Filter to enhance a photo-
1. In the Develop module, select the Radial Filter tool from the toolstrip.
2. Do one of the following:
• To create a Radial Filter, click and drag the mouse across the region of interest. This will draw an elliptical shape, which determines the area that is either affected or excluded from the adjustments you perform.
• To edit an existing Radial Filter, click any of the gray handles on the photo.
While drawing, press Shift to constrain the Radial Filter to a circle.
3. To determine what area of the photo is modified, select or clear the Invert Mask checkbox. The checkbox, by default, is not selected.
• Invert Mask not selected(default): Changing any setting affects the image region outside the marquee area.
• Invert Mask selected: Changing any setting affects the image region inside the marquee area.
4. Adjust the size (width and height) and orientation of the Radial Filter added. Select a filter, and:
• Click and drag the center of the filter to move and reposition it.
• Hover the pointer any of the four filter handles, and when the pointer icon changes, click and drag to change the size of the filter.
• Hover the pointer close to the edge of the filter, and when the pointer icon changes, click and drag the edge of the filter to change the orientation.
5. Use the adjustment sliders (shown in step 1) to create the desired visual changes. Use the Feather slider to adjust the visual falloff of the applied adjustment.
6. Repeat steps 2 through 5 to continue adding or editing filters.
7. Click Reset, to remove all the Radial Filters applied to your image.
Keyboard shortcuts and modifiers for the Radial Filter tool
• Press and hold Shift + drag, to create an adjustment that is constrained to a circle.
• While dragging one of the four handles to resize an adjustment, press and hold the Shift key, to preserve the aspect ratio of the adjustment shape.
• While an adjustment is selected, press the Delete key to delete the adjustment.
Adjustments with maximum coverage
• Press Command/Control and double-click an empty area, to create an adjustment that is centered and covers the cropped image area.
• Press Command/Control and double-click within an existing adjustment, to expand that adjustment to cover the cropped image area.
• Double-click without pressing the Cmd/Ctrl key commits and dismisses the Radial Filter tool.