XM 70 FR is the front Hydractive sphere, mounted under the car, behind the engine.
XA 50 AR is the Hydractive sphere mounted under the rear of the car between the rear subframe and exhaust transverse silencer. Your car probably has a heat shield mounted in this area, to protect this sphere from exhaust heat.
The main accumulator provides a reservoir of pressurised fluid for the system to draw from.
The Hydractive spheres provide additional gas volume, so softer suspension, when the system is in soft mode.
Have a look through the material in the linked thread, to fully understand how the system functions. viewtopic.php?f=13&t=10291
Picture of member entirelyturbo changing his rear Hydractive sphere.
You should be able to Unbolt and unclip the heatshield and push it over the exhaust, so access the sphere with the exhaust still fitted. It's best to remove the spare wheel carrier to create working space.

