The Capture Flap is a PALS cut, beaver tail style accessory that can mount to any HPG pack and packs of other manufacturers. You can mount PALS pouches on the front, and either PALS or velcro pouches on the back.
The Capture Flap is mounted via non-adjustable G-hooks on the bottom that connect to the webbing tabs on HPG packs. The top of the flap comes with an adjustable amount of webbing on G-hooks to loosen or tighten the flap against the pack. Any location that you can connect 4 G-hooks to is a potential candidate for the Capture Flap.