After saving a selection (Application menu > Select > Save Selection), a layer with said selection is visible in Channels (Layers panel).
Is it possible to modify this selection in a future stage? For example in the Select and Mask workspace.
You can also modify the channel directly form the Image > Adjustments menu, such as with Levels or Curves/
I find it best its best to add the color channel, or alpha channel to a layer as a layers Mask. With the layer mask targeted adjust the layer mask Ctrl+L levels or Ctrl+M curves. When you make the curves or level adjustment you can observe the effect the mask is having on the composite image.