Yes they can be a slot or a series of holes.
When the windows are made they'll use a machice similar to a router to form the slot but to retro fit them its way easier just to make holes. The only problem i can see is if your windows are steel reinforced, its not easy drill through steel when your perched up a ladder drilling inwards, i speak from experience
To fit them i would offer one up to the inside of the window, centralise it and make sure its level. Then mark underneath it where the 2 outermost holes would go, (obviously they have to be within the coverage of the vent), remove vent and start drilling 20 holes, the 2 outermost holes drill right through to the outside, this means you can nip out and draw a line between the 2 holes to use as a guide, the last thing you want is to drill all 20 holes from inside only to find the outer holes are all over the place.
Good luck