Good morning Alessandro,
-Softer compounds are more suitable for sealing rough surfaces, as their flexibility and malleability means they are able to adapt to irregular areas allowing a hermetic seal
-Harder compounds provide greater resistance to abrasion and extrusion but offer less flexibility making them the best choice for sealing even areas subject to high pressure
Most polymers used for seals fall within the 60-90 range of the Shore A scale, as they offer the necessary resistance to extrusion and pressure
The standard for most seals is Shore 70A Hardness as it's the ideal combination of malleability hardness and durability, able to flex when necessary but also withstand pressure
Seals with 90 Shore A Hardness are harder, able to withstand more pressure. These materials are more durable and can be used in dynamic applications
I hope this explanation is helpful to you
Best regards