Peak Design Capture Pro is a device that enables you to quickly mount your camera in various positions such as on a backpack strap, belt or bag
If you get the Pro Plate you have to screw two little adapters into the side of the plate so it will fit a Manfrotto RC2 head. Without the adapters it is an Arca Swiss only plate. Putting the adapters on is only useful if you have a few Manfrotto RC2 heads around that you'd like to attach your plate to. You don't have to put the adapters on if you don't need them, so I guess the logic at Peak Design is that you get the best of both worlds with the Pro Plate and they don't have to make different plates. You should note that the Pro Plate won't work with other manufacturer's own brand quick release systems.
Once the adapters are on, although it means you can mount the plate on both an Arca Swiss head and an RC2 plate, you can only do it from two directions instead of four directions because the plate will no longer be square. You might find this to be inconvenient. Also it won't work with all RC2 heads.
One thing you can do is to convert your Manfrotto heads to Arca Swiss which is the way to go in my opinion but this might get expensive if you have quite a few RC2 heads around. If you've only got one or two then you can buy either a very inexpensive Chinese Arca Swiss plate, use the reasonably priced FotoPanda (Desmond in the USA) plate or go expensive with a Really Right Stuff plate
I might really take to using the Capture Pro or it might just end up being something that gets put in the back of my gear drawer and pulled out now and again.
It will be something I can also use with my Go Pro and if I buy the new Peak Design Lens Kit that works with Capture it will be very useful indeed. In fact the Lens Kit is something I can really see myself using.
Mounting the Capture Pro to the top of a monopod might be a cool way to use it too. I won't be able to do portrait orientation with it if I stick it directly on the monopod. I don't currently need to be able to screw it to my monopod but it's handy to know it can be done and I might find it a useful way of using it.
For more information about the Capture Pro check out the manufacturer's website