By Custom filter you mean what appears from a Right Click on a Thumb and then CUSTOM FILTER?
Like This?
It is a fast way to Customize your current filter that you did up in The Image editor, or tweak a number such as  DFS Radius from a 4 to a 5 , or a tick more on the Contrast,,,,
Or you can just create a new flt filter by adding something that you think it needs, and be able to do it without the need to actually open the Editor screen.

A mid image "quickie" for you adjustment finesse package.
(Loads of things you can do and they are instantly applied)

Suppose, you were all done editing and as you looked before you printed, and you saw, The Dreaded NOISE PIXELS!
You open that Custom Filter, tick the Noise reduction filter box, DONE....
Fred