If you look to the right and slightly above the line that reads Tmax, it says "T= Wall Thickness". Since plate does not have 'Wall Thickness' it would seem that the T is equal to the wall thickness of the tubing. Thus, T Max for plate where the arrows point to the plate thickness would indicate that the plate thickness is a MAXIMUM of the tubing wall thickness dimension.
So, if you are using a pipe with a .22 wall thickness, your plate would be 3/16" since 1/4" would be over your maximum of the tubing wall thickness. Of course, you could always waste your time and take a 1/4" plate and machine it down so that it was equal to the tubing wall. But why? Just a thought in case anyone was thinking about it.
He Is In Control, Have a Great Day, Brent