I'm carrying a Maxpedition Spartan.
Velcro tabs hold the 3 card slots on the left shut on one side, and 3 plus the second bill pocket shut on the left. You can turn the wallet upside down and shake it, nothing will fall out!
Two back slots for bills, I usually use the one up front for receipts. The transparent mesh is nice when you go to find something, and it's a lot more durable than it looks.
Same mesh is on the ID card slot on the back. A lot of wallets use a PVC window for an ID slot, which always discolors, deforms or starts rubbing ink off the ID. Not a problem here!
The whole thing seals shut tightly with Velcro.
The only complaint I have about the wallet is--well, that velcro is pretty geeky--and that all of the pockets are tight. This is a good thing, but it sometimes makes slipping a wad of bills and receipts in and out difficult.
It's also a little thick, but it's a lot more comfortable than other wallets I've had.