SP stands for Service Pack 1 or 2, which are patches improving FSX. They are available after the game’s release.
Flight Simulator X Service Pack 1 (Click to download) corrects the following issues found by clients after the game’s release : • Activation and Installation • Problems linked to the use of additional components • Improvement of performance • Problems linked to the content
Flight Simulator X Service Pack 2 (Click to download) improves the following functions : • Multiplayer compatibility • DirectX 10
You have to install the Service Pack 1 (SP1), before the second one.