A radial mask is parametered by a 2D ellipse and enables to define a radial selection.
Note that similar and more evolved masks can be obtained by combining a
Turn any 2D generator into a mask
This generator creates a linear gradient (by default from black to white), in a circle from the center to the edges of the layer
Related masks in 3D space are:
Mask with a sphere shape
Mask with a cylinder shape
If you want to create a perfectly round radial mask, put Smode in pixel unit and there you go :