The two basic types of Assembly design:
Axial “open” groove design and Closed groove.

Both types are used on the piston side as well as on the rod side.
The O-ring is the most cost effective and convenient sealing element available to the user.

Some sealing elements like seals with large cross section need an open groove due to it being stiff and needs an open groove to achieve simple assembly.

Open grooves are also required in cases where assembly involves small diameters.