Ed,
There is a layer of perforated film between.
The holes must have been poked through by a machine, very small and very close together, but one side is just a little more rough than the other side.
It can be very hard to tell which is the smooth and rough side because can't always feel it with your fingers tips.
When it is like this I rub it against my lips to work it out.

Right near the end of the project I found out that if it is laid on the peel ply with the smoother side down it will stick to the wet resin really well and the rougher side will be enough to hold the absorption fabric in place on vertical surfaces.
Also this way around the perforated film comes off the peel ply after cure very easily with the absorption fabric. The other way around and it won't stick and hold he absorption fabric so I had to come up with other ways of getting everything to stay in place. Also it stays with the peel ply
when the absorption fabric is removed.
Had me going for a while trying to work out why some sessions were just so much easier than others when it came to placing the consumable layers and removing them after. When I bought the laminating materials I had to buy a full roll of this perforated film and that was 1000 metres. Luckily it was quite cheap.
Regards,
Phill