An AIP is not a file, but a special directory. It is created by extracting the contents of a self-contained MSI-file into a previously empty directory with the command 'msiexec /a'.
An AIP typically contains another, much smaller, MSI-file, and some other files and/or subdirectories. This includes often one or more CAB- and/or MSP-files.
A AIP can be used equivalent as a self-contained MSI-file. Just select the MSI-file that is included in the AIP. The Windows Installer finds and uses the other files, if they are in the same directory as the MSI-file. If the AIP contains subdirectories, you must preserve the whole directory structure.
To deploy an AIP via GPO, copy the whole AIP-directory to the server and specify the MSI-file contained therein in the GPO.
Usually there's no advantage of using an AIP instead of a self-contained MSI-file. But sometimes when you extract a MSI-file from a installer of kind setup.exe, you just won't find a self-contained MSI-file, but you get an AIP instead. An example for this is Java. AIPs are also created when you apply a patch from an MSP-file to a MSI-file. You need to do this for example if you want to deploy a slipstreamed minor version update of Adobe Reader via GPO.