Android system里面的签名

Besides the value platform and shared, there are other two values called media and releasekey. The following list illustrate their differences which was copy from Android build system from here.

-platform: a key for packages that are part of the core platform.
-shared: a key for things that are shared in the home/contacts process.
-media: a key for packages that are part of the media/download system.
-releasekey: the default key to sign with if not otherwise specified

Those are the signatures used by Android security system to ensure the core component and the framework of the system's security is under the control of manufacture.
The link about Android build system I gave above is not official, but it's a good material for references.

Update:
You can also set LOCAL_CERTIFICATE to be PRESIGNED, which tells the signing script that this APKs are already signed and should not be signed again.