The Plant/P&ID SP contains all of the same fixes that are in the AutoCAD SP, plus specific fixes for P&ID and Plant 3D.
Important note for Autodesk Subscription customers:
If you plan to install AutoCAD Plant 3D 2015 Extension 1, you must do so before installing this service pack. You can install AutoCAD Plant 3D 2015 Extension 1 before installing AutoCAD Plant 3D 2015 Service Pack 2. If you have already installed AutoCAD Plant 3D 2015 Service Pack 2, you can uninstall it. Afterwards, you can install both the extension and the service pack (in that order).