A ZIP code looks like a place but is not one. It describes a mail delivery route, and treating it as a boundary produces predictable errors.
The code encodes a sorting path
The digits are hierarchical. The leading digit identifies a broad region of the country, and subsequent digits narrow toward a specific processing facility and then a delivery unit.
That structure exists so that mail can be sorted progressively, with each stage of the network reading only as much of the code as it needs to pass the item onward.
The design goal was throughput. Nothing in it required the resulting areas to be compact, contiguous or stable, and often they are none of those things.
Some codes cover no area at all
A large building receiving enough mail can be assigned its own code, which then describes a single address rather than a neighbourhood.
Post office boxes at a facility may carry a separate code from the streets surrounding it, so two codes overlap the same physical ground entirely.
Rural routes stretch codes into long irregular shapes following roads, because the code follows the carrier's path rather than any settlement pattern.
Boundaries move without notice
Codes are adjusted when delivery volumes shift, when facilities are consolidated, or when new development changes the workload on a route.
Those changes are operational decisions, not administrative ones, and they do not align with the redrawing of any other boundary in American life.
An address can therefore change its code without moving, which complicates any record system that stored the code as a permanent attribute of a location.
They do not nest inside other boundaries
A single code can span two cities, two counties, or two school districts. It has no obligation to respect any of them.
This creates persistent trouble for anything that assigns obligations by geography, since a code is not a reliable proxy for which jurisdiction an address sits in.
Sales tax administration illustrates the problem clearly: rates are set by jurisdictions whose borders cut through codes, so code-level rates are approximations.
Statistical use requires a substitute
Because codes are routes rather than areas, they cannot be used directly to publish population data. Statistical agencies construct approximate area versions instead.
These approximations are built from the codes appearing at addresses within small census units, producing polygons that resemble the codes without being identical to them.
Analysis using those areas therefore carries an inherited approximation, which matters most where a code is unusual in shape or where its boundary shifted between updates.