Resolve a US ZIP code to the census tract(s), county, CBSA (metro), and congressional district it covers — each with HUD-USPS allocation ratios (by residential, business, other, and total address counts). A ZIP is a mail-delivery route set, not a polygon, so it spans multiple geographies; the ratios tell you how much of the ZIP falls in each — the canonical join when you have a ZIP but a dataset is keyed by county/tract/district. Source: HUD-USPS ZIP Crosswalk (public domain).

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