When I right click a book in my Calibre library, I see in the menu, "Save only MOBI format to disc". I'd like to change this so it says "Save only AZW3 format". I know that I've changed this setting in the past, but I've been right through the "Preferences" pages and I'm damned if I can see where it's set

. Could someone be kind enough to remind me, please?
Note: It's not the "Preferred output format" on the "Behaviour" page. That sets the default format for conversion, but not for saving.