Pre-Process
The pre-process section lets you set a threshold for controlling where the Staglow effect appears and a built-in circular mask for restricting the glow area. The Pre-Process interface in After Effects.
Threshold
The threshold defines the minimum level for the highlights. A low value will give more glow in the image, and a high value will make only the very bright areas glow. The images below show the output of Starglow using a shot of the sun on water with different Threshold values.

Source
High Threshold
Low Threshold

Threshold Soft

This controls sets the softeness of the border between lowlights and highlights.
Use Mask
Check this to enable the internal circular mask.
Mask Radius
This control sets the size of the internal circular mask in pixels.
Mask Feather
The Mask Feather value sets the softness of the internal circular mask.

Mask Postion
You can set the center point of the circular mask with this control.