It looks similar in function to the adapter that I added on my kubota to add rear hydraulics. I removed a cover and replaced it with a fitting that is different looking than the thing you have, but similar in function. The adapter I added then provided two lines, one high pressure to rear valves, and one return. I would guess yours does the same, you would connect the P to your rear hydraulic valves, and the return from your rear valves to the T port. From what others have said the R must be a relief, and I would guess that if your rear valves/implement has a relief valve, then you would connect the return to the R port rather than the T port. On my tractor if the rear valves (or intended implement) have a relief valve, I'm supposed to plumb them to a different place on the tractor that goes directly to the tank. I think this is to avoid exposing the normally low pressure return side to high pressures resulting from the relief valve going off. So if I had to take a guess on how to plumb that, I'd plug the port labeled T, and connect the return line from your valves to the port labeled R. That way if you use an implement that has it's own relief valve, you never have to worry about it dumping into the low pressure side of your system. The down side of doing that is I think you bypass the filter on the return side, but I'm not sure about that. You might take that plate down to your local dealer, assuming you're not out in the boonies.
