We'll be the first to admit, it is confusing; made worse by the fact that the abbreviations are often misapplied and misused. The meanings are slightly different depending upon whether you are referring to hoses or first stages. There are two broad categories of SCUBA hoses; high-pressure hoses with service pressures up to 5000 psi and low-pressure hoses with service pressures up to 250 psi. The first stage regulator has two types of ports to which these hoses may be attached; high-pressure ports directly supply the tank pressure and low-pressure ports which supply a regulated pressure relative to ambient of about { 135 psi | 9.5 bar}. This lower supply pressure is often termed the intermediate pressure because it is between the cylinder pressure and the ambient pressure; or looking at it another way, between the first and second stages. These pressures, hoses and ports have different names and that is where the abbreviations are used and the confusion starts.
The easy part: both the high-pressure hose and the first stage regulator 7/16" port it connects to are abbreviated "HP". There are also two types of hoses that connect to the low-pressure supply from the first stage. Hoses that connect to a second stage regulator are abbreviated "LP". Hoses that connect to buoyancy control devices and drysuits are abbreviated "BC". When talking about hoses then "LP" and "BC", while both connect to any low-pressure supply port on the first stage, at the other end each connect to different devices with different fittings.
Many regulator manufacturers stamp an identification beside the ports on the body of their first stages to make it easier to identify the service pressure the port supplies, high or low. First stage regulator ports are typically stamped either "HP" or "LP" if they are marked at all. However, some manufacturers instead stamp the low-pressure port "IP". When talking about a first stage then "LP" and "IP" mean exactly the same and are the same 3/8" size port.
Along comes Apeks, who advocated a new 1/2" size for an optimized flow port on their first stages. Apeks started making their first stages with two different sizes for their low-pressure ports, the near universal size intermediate pressure 3/8" IP port and what they called the middle pressure 1/2" MP port for the divers primary second stage. Apeks has since deprecated the 1/2" port but they have sold a LOT of first stages with that oversized MP. Just to be clear, intermediate pressure (IP) and middle pressure (MP) are exactly the same low-pressure supply delivered from the first stage.
7/16" Port = HP, 3/8" Port = LP or IP, 1/2" Port = MP