Along with this unified playing experience, crossplay enables cross-progression, which means your progression in Modern Warfare II will carry over across all platforms linked to the same Activision account, regardless of console generation.
This can be done by entering the credentials of PSN, Xbox, Steam, or Battle. Click on the profile name at the top right of the screen and select the "Linked accounts" option from the drop-down menu. Click on the Gaming Networks that require linking through which the progress will be saved across platforms.