This is a pretty slick little pack/back panel to integrate into the D3CR.
It is pretty small - 1 liter hydro capacity, 700 cubic inches when fully expanded - but is sized well to fit onto the back of a plate carrier. Tactical Tailor/Grey Ghost Gear and others have launched backpacks designed to integrate into the back of an armor carrier, but they mostly seem to end up a bit big, bulky and unwieldy.
A compressed FlatPack looks about as slick as you can hope to get for a hydration carrier, yet it gives you the ability to expand out and carrier more gear if needed.
This isn't for carrying a multi-day load for sure, but enough space to support a day at the range, a long hike or a short range patrol activity.
I have a D3CR, and the chest rig harness is a bit on the lame side. That makes the FlatPack all the more appealing to me, given that it's is intended to replace the harness and integrate in with the chest rig.
The price may be a bit rich for a small, fairly basic pack ($130), but it's got a unique set of capabilities that don't exist elsewhere.