What is the Android GSF package?

What is it the GSF_PACKAGE I found in the C2DM Tutorial?

To be precise:

public static final String GSF_PACKAGE = "com.google.android.gsf"

It was used in intent:

registrationIntent.setPackage(GSF_PACKAGE);

On the developers site I found this:

"setPackage(String packageName) (Usually optional) Set an explicit application package name that limits the components this Intent will resolve to."

Anyway, what limits and why they might be needed?

gsf = google services framework