The difference between high pressure and low pressure LP is this. High pressure is rated in PSI (pounds per square inch) of pressure. This is like the pressure in your car tire. The typical high pressure regulator is set around 10 PSI.
Low Pressure LP is rated in inches of water column (in WC). The typical rating for low pressure LP is around 10.5 - 11 inches WC.
Water column is determined by this. Take a U
shaped container. put water in the container until the bottom is filled on both sides. Mark this point 0. On one side put a ruler on the container. Now you have what is called a graduated cylinder.
On the other side blow air down until it pushes the water up the graduated cylinder to the 11" point. The amount of pressure it takes to do this is 11" WC. Less than the amount of air in a balloon. Less than one stroke on a bicycle pump. Not much pressure another words. Natural gas is about 3.5" WC. Damned near no pressure.
Don't know if this will help or not, but that's what the difference is.