IEER — the Integrated Energy Efficiency Ratio — is the efficiency metric for commercial unitary air conditioners and heat pumps (those at or above 65,000 BTU/hr). It's the commercial counterpart to a home system's SEER2. Unlike the older single-point EER, IEER is a weighted average of efficiency at four load points — 100%, 75%, 50%, and 25% — which matters because a rooftop unit spends most of its life at part load, not running flat out.
Federal minimum efficiency standards set IEER floors that step down as equipment gets larger. For air-cooled commercial package AC manufactured on or after January 1, 2023, the minimums are 14.8 for units from 65,000 to under 135,000 BTU/hr, 14.2 from 135,000 to under 240,000 BTU/hr, and 13.2 from 240,000 to under 760,000 BTU/hr (10 CFR 431.97). Efficiency is verified by testing to AHRI Standard 340/360.
For a building owner, IEER is the apples-to-apples number when comparing replacement quotes. A unit that just clears the federal floor and one rated several points higher can carry a real operating-cost gap over a 15-year roof life, and the higher-IEER tiers are usually what utility rebate programs require. Ask for the AHRI certificate on the proposed model — the IEER and the model number on that certificate are what a rebate reviewer (and any honest comparison) will go by.