Am I the only one who did not know about this?

[R C-R]

While editing an old eps file I had imported into Affinity Designer, I tried to use a Boolean Subtract to convert 3 curve layers to a single Curves layer with 2 transparent holes in it. I have done this a zillion times without problems but for these 3 curves what I got instead was no transparency, something like this:

It took me much too long to figure out why, but the reason is simple: by default, shapes are assigned the Alternate (Even-Odd) fill mode. For whatever reason, the shapes in the old eps file all used the Winding (Non-Zero) fill mode, which for subtractions like this one fill everything.