An MSP-file contains a patch to be applied to an installed software product, or to an AIP.
A patch is usually an update to a new version of a software product.
A typical example for MSP-files are the updates for Acrobat and Adobe Reader provided by Adobe.
Creating a patch file is an alternative to creating a new MSI-file.
A patch from an MSP-file can be applied to an already installed software product
by double-clicking the MSP-file, or by issuing the command
msiexec /p patch.msp REINSTALL=ALL REINSTALLMODE=omus
(the options are important, otherwise it only updates the locally cached copy of the MSI-file).
It is also possible to let windows installer install an MSI-file
plus one or more MSP-files in one step with a command like this:
msiexec /I package.msi PATCH="c:\directory\patch.msp"
In theory you could also just apply the MSP to the installed version with a GPO startup script, instead with GPO software deployment, but then you must do the version management for that software yourself. The script runs each time when the PC boots, so it must check which version is already installed. Also repair and uninstall probably become tricky, because you have a later version installed, than noted in the group policy. So you should probably not just deploy the MSP updates with the script, but the base version MSI of that software as well.
It is possible to slipstream one or more patches from MSP-files into an AIP (for example one extracted from an MSI-file), and to deploy that with GPO.
Slipstreaming requires these steps:
If you don't want to do this manually, you may use my batch ApplyMSP.bat. Simply create a new directory, and copy the MSI-file (or an AIP), the MSP-file, and this BAT-file there. Then double-click on the BAT. It will display the name of the MSI-file, and of the MSP-file that it has found. Then it asks if the MSI is a self-contained MSI, or part of an AIP. You must typein either 'MSI' or 'AIP' and press the enter key. The result of the slipstreaming will always be an AIP. Thus if there is more than one MSP to be applied, you must copy all but the first into the AIP directory, one at a time, and apply them there (also copy the BAT file into the AIP directory and double-click it there). I'm using this for Adobe Reader.
News: try the new version ApplyMSP2.bat. It detects itself whether it is processing a freshly extracted MSI, or an already slipstreamed AIP. It simply checks whether or not a file Data1.cab still exists. If yes it is processing a freshly extracted MSI. If not, it processes an AIP into which already another MSP has been slipstreamed.