An O-ring seals by being squeezed — compressed slightly smaller than its free cross-section when the groove closes. Too little squeeze and it can't maintain contact as things vibrate or wear; too much and friction, compression set and heat build-up shorten its life fast. The groove also needs to leave room: gland fill is how much of the groove volume the ring actually occupies, and it needs headroom for thermal expansion and fluid swell.
A 3.53 mm cross-section (AS568 "139" series) static seal targets about 20% squeeze — a groove depth of roughly 2.8 mm. Switch the same O-ring to a dynamic (sliding or rotating) application and the target squeeze drops to around 12%, giving a deeper groove and less friction — the trade-off dynamic seals always make against static ones.
Squeeze is what makes the seal — but it's also what makes friction. A static seal never moves, so it can afford more squeeze for a more robust seal. A dynamic seal (a rod, piston or rotating shaft) pays for every percent of squeeze in friction, heat and wear, so the design target is deliberately lower.
Above about 90% fill, the O-ring has nowhere to go as it heats up or absorbs fluid (swell) — it can hydraulic-lock in the groove, spike contact pressure, and extrude out through any clearance gap. Below about 70%, the ring can shift, twist, or fail to fully occupy the groove under pressure.
A face seal is squeezed between two flat surfaces closing together, like a lid — simple to design and assemble. A radial seal is squeezed between a shaft and a bore, sealing sideways — the standard choice for anything that slides or rotates, like a piston or rod.
A dash number just pairs a standard cross-section with a standard inside diameter — the groove design itself depends only on the cross-section and seal type, not the specific ID. Entering your own ID here gives an exact, correct groove for any size, standard or custom, without needing to look up a dash-number table.