In plain language
Instructions must not rely only on shape, position, size, or sound ('click the green button on the right').
Who it affects
Blind, low-vision, and color-blind people when instructions mention only appearance or sound.
How to check
Read every instruction and ask: would it still work if you couldn't see the layout or hear the sound? Each referenced control must be named in text. Worked example — A calendar app's event-creation form: read the instruction alone: 'when you hear the chime, proceed' relies purely on sound — check for a visual/text equivalent for deaf users.
Common misconception
“Click the blue button on the right” feels harmless to a sighted reviewer, but it gives a blind user nothing to act on. Every reference needs a name, not just a location or color. Real example: on a music streaming app's playlist screen, instructions identify something only by its shape or position. 'click the round button' or 'use the box on the right' — which is meaningless to blind users or when the layout changes.