The gauge pressure is the pressure shown by a gauge which was calibrated and zeroed using ambient surface pressure at sea level as a reference.
SPG or Submersible Pressure Gauge is a gauge that translate the pressure from the tank to a unit that has measurable properties. Similar to the fuel gauge of a car. They can either be in PSI for imperial system or bar for metric system. They are the same as their regular non-submersible counterpart but sealed to block water from getting into the mechanism preventing corrosion.
The absolute pressure is compared against a perfect vacuum. It will read the actual surrounding pressure plus the pressure of the atmosphere. It is used in technical or altitude dive planning. The absolute pressure is not the pressure from the submersible pressure gauge as it represents the gauge pressure.
A burst disc is a safety feature of cylinders valves to help prevent overpressurization. It will as the name implies burst with the excess pressure and drain the cylinder until it is at ambient pressure.
The LPI or Low Pressure Inflator allows the BC or BCD to be inflated or deflated. The LPI is fed by its own hose connected to a low pressure port of a regulator first stage. There are only two buttons, one for inflation and the other for deflation.
The PSI or Pounds per Square Inch unit is from the imperial system of measurement and is used to express the value of pressure. Some countries such as the US still use it but most countries in the world use the bar which is the metric unit for pressure measurement.
HPNS or High Pressure Neurological Syndrome occurs on very deep dive usually past 100m and it affect the central nervous system. Syndromes include tremors, spasms, paralysie. Little is know on the physiological mechanics of HPNS.