bottom, many manufacturer's believe they need to replicate the " communication holes " bees tend to make in comb. I think it's ridiculous, while they do it occasionally not so often to manufacture it into a product.
I think these corners are for the bottom grooved bars that have the groove sloped up at each end, I had some of those frames and had to cut the corners off to have them work, I use rite-cell sheets from mann lakes and they come with the corners attached so you have the choice to take them off or not. and it is corners down
I would suggest a location specific kind of thingee. The notch of course goes to the bottom side of the frame.
In warmer climates I suspect the 'communication' passage is of little value. In colder climates this provide a mechanism for bees in their winter mode to move from one side of the frame to the other. A long time ago in doing some experiments in keeping bees in extreme conditions hives constructed without these 'holes' would not survive a winter no matter how well provisioned.
I ordered a few split bottoms just to see what they looked like and found what Zookeep observered. I put them together with wax foundation and had to trim a few bottom corners. I also got some of the black plastic foundation with the perforated corner tabs and figured that communication holes if anything should be more use at the top of the frames so I nipped them off, and that is where they are.
I do see peek a boo holes in the wax but never made note if there is a pattern to where they usually put them.