STATIC ELASTOMER S084
In this drawing The static elastomer (O-ring) seals the mechanical seal inner sleeve to the shaft.
The dynamic elastomer is the one located between the seal inner and outer sleeve.
Most mechanical seal designs incorporate both dynamic and static elastomers. Dynamic O-rings are required to flex and roll with the shaft movement. This means that a very low shaft squeeze is important to prevent seal hang up or hysteresis.
They must also be free to flex and roll to compensate for mechanical seal face wear.
Static O-rings do not have to move. They are often used as a gasket and are a lot more forgiving than dynamic O-rings because a small amount of swell can be tolerated that might even improve their sealing.