The inside diameter is used to describe rotationally symmetrical bodies such as rings and hollow cylinders. They have a passage, the size of which is determined by the inside diameter. The outer diameter is calculated from the inner diameter and wall thickness.
Sealing rings are usually installed in circular grooves, so the diameter is crucial for the correct fit of the components.
For O-rings, it is therefore common to specify the inner diameter and cross-section (cord thickness).