The meaning should be self explanatory. The HD oiling system does not require high pressure to provide lubrication where needed. The system relies on volume.
If you have ever adjusted lifters on a small block Chevy, you will remember how the oil squirted in your face and everywhere else. Pull the rocker box cover on a TC motor at idle and you will see the oil pumping up through the pushrods to lubricate the rocker arms but very little squirting; that should explain the difference.:s
Of course the system requires pressure to move the oil but the HD lubrication system does not require the same pressure that a typical V8 system requires to lubricate cam bearings, rod bearings, etc.
IMHO, you gain nothing by having 20psi at idle over having 10-12 or having 50psi at cruise over having 32.:s As a matter of fact, higher pressures can cause aeration in the oil which can negatively affect lubrication and valve train noise.