Spring-energised metal O Ring for Internal pressure (OSI) and (OSE) for external pressure are designed for extreme low leakage at moderate pressures.
Internally (OVI) or externally (OVE) vented O Rings are designed for higher pressure. Besides the own seating load the seal is also energised by the system pressure.
Non-vented rings (OI & OE) are designed for lower pressure ranges. The perfectly closed O Ring design avoids ingress of the media into the seal. Moderate load, moderate pressure capability.