1. A given land parcel may not have mail delivery (think: rural land, even with people living on it), so... what is its zip code? Does it even have a zip code? Remember, a zip code is defined as a set of mail delivery points.

2. A given land parcel may have multiple mail delivery points, each of which may have a different zip code (think: a large mall or commercial building with multiple tenants, where it may be more convenient for the USPS to deliver from two or more mail routes). Does that land parcel have more than one zip code or do you arbitrarily pick one? If it has more than one, does the whole parcel have both codes (so, overlapping polygons), or do you try to subdivide it somehow?

[Users of zip code polygons often want to preserve the capability to look up the zip code from a given lat/lng point. So they will yell at you if you choose to have overlapping polygons or two zip codes for one polygon.]

3. What about AFPO addresses that represent the current location of a group of people who move around all the time? An aircraft carrier out in the Pacific is not near any land parcels.