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.
A generator can also mix modes within a single QR code, switching between numeric, alphanumeric, and byte segments as needed to encode each part of the content as efficiently as possible — a URL with a long numeric ID at the end, for example, might encode the alphabetic domain portion in byte mode and the trailing digits in numeric mode, though this level of optimization happens automatically and isn't something a user configures.
Alphanumeric mode's limited symbol set (space, $, %, *, +, -, ., /, and :) was chosen specifically to cover common uses like encoding uppercase serial numbers, ticket codes, or simple structured identifiers, not general text or web addresses. That's a large part of why it sees far less real-world use today than byte mode, even though it's technically more space-efficient when the content happens to fit its restricted character set.
Kanji mode exists as a fourth option purely for efficiently encoding Japanese kanji characters, reflecting the QR code's origin at a Japanese company. It has essentially no relevance to English-language content and is invisible in practice for the vast majority of QR codes generated worldwide today.