Hi Philip,
I'll try my best :
Screenshot of the mass action is attached, there is really not much to see other than that
I have a list of products who have no brand and I want to add a brand to them using mass action.
In the product listing I select one or more products, go to mass actions and choose to edit the categories.
Then choose "add" and select the brand from the tree of brands.
Then click the process button.
I would have expected the brand to be added into the field "Brand" but it is added to the field "Categories".