The X-ring is a double acting rubber seal with four seal lips two dynamics and two static or you can say four static if a static application.
X-ring is used for low pressure system in both hydraulic and pneumatics systems. The X-ring is sometime used in rotating application with low pressure. The advantage with the X-ring is that it have low friction and high seal ability due to the four sealing points, also possible to keep lubrication between the sealing lips. This will make the X-ring very stable in dynamic application.