to be pendantic, all (US) ZIP codes are just for routing. Any correspondence to a geographical area is an artifact of this, and subject to change. Also, it is possible for two ZIP codes to overlap geographically.
In particular some numbered zones are entirely contained geographically within others. In some places in the US, there's a ZIP for delivery to private homes and businesses but another for delivery within the Postal Service network, including delivery to post office boxes. The one I remember most readily is Quincy, Illinois. The city is 62301 and the surrounding county 62305, but PO boxes are 62306, located within the post offices that sit within the 62301 zone.
