When making local adjustments in Darktable, one of the ways to isolate a part of an image is to use drawn masks. These can be combined in a number of ways, more than you can accomplish directly in the module where you make them. They can also be reused in other modules, and organised in different ways. The masks can even be named, so you can easily see which is which.
When you have just started out with Darktable it may not be that obvious. You see the drawn masks in the different modules, but there’s a limit to what you can do with them. Now take a look at the left side of the screen and you’ll find the Mask Manager. Starting to use this will open up many new possibilities for your drawn masks.
It is important to keep in mind that the Mask Manager can not handle parametric masks. These are always dependent on current pixel values, so they can only be used when and where they are made. That said, I think I saw somewhere that the ability to store parametric masks may come at a later date.
But for now it is all about your circles, ellipses, patches, brushes and gradients. View the video below and learn how to handle them like a pro.