Spring center just means what it says. When you let go of the lever it goes back to "center" .
Floating detent valve is like the Spring center valve with one added feature. IF you push the level completely forward it will detent into a float position.
Self canceling Detent valve: This is a valve where you can put the lever in the up or down position an it will stay there UNTIL the cylinder completes its cycle. Then the lever will return to neutral. It also will have the detent for float in it.
They are listed in the order of increasing cost too. The spring center valve would be the cheapest one and the Self cancelling the most expensive.
I would assume that the Front loader with QC would mean that the tractor would come with a Quick Change loader installed.
The full loader package would be that the tractor had the hydraulics and maybe even the mounts for a loader that the dealer already has in stock. So basically the tractor is all ready for a loader but it is not mounted on it.
This is how JD and Allied loaders used those terms. So I would think that Kubota would be very similar. Much of this you should be able to figure out just by cost. The Loader with QC should be a higher price than the tractor with just the "full loader package".