I cannot find any easy way to work with saved selections in Krita. What I would like to do is to save selections in some list, and when needed combine/subtract some of them to create a new selection temporarily I could work with. There are several ways to "save" a selection in Krita which I am aware off:

• create a separate opaque layer, paint a future selection and then later in the drawing process I can select that layer and Select->Select opaque to "restore" a selection. I can add another selection to it by choosing another layer and doing Select->Select opaque. But I could not find a way to subtract one selection from the other. The advantage is that you can restore a selection even if layer is not visible.
• create a local selection. The disadvantage is that these are per layer, and I could not find a way to subtract one from another either.

Is there any solution or workaround for that?

My workaround is like that:
1. select opaque from layers A, B as you described
2. to substract layer C selection, I switch to layer C and just use a magic wand (contiguous selection tool) set to Substract. Woudln't work smoothly if the layer isn't flat color though.

Substraction of opaque selections would be a nice feature to have, I would support it