QR Code Alphanumeric Mode
Alphanumeric mode is one of four ways a QR code can encode data — it covers digits 0-9, uppercase letters A-Z, and a small set of symbols (space $ % * + - . / :), and packs that content more efficiently than plain byte mode, but it can't represent lowercase letters.
The QR spec defines four encoding modes: numeric (digits only, most efficient), alphanumeric (uppercase + limited symbols), byte (any character, including lowercase and Unicode), and kanji (optimized for Japanese characters). Most real-world content — including nearly every URL, which relies on lowercase letters — can't use alphanumeric mode's efficiency gain and defaults to byte mode instead.
This is invisible to anyone using a generator day to day; the software picks the correct mode automatically based on what's being encoded. It only matters if you're troubleshooting why a code came out denser than expected for a seemingly short input.