MUST, SHOULD, and MAY for apps

UX requirements are expressed as instructions that you MUST, SHOULD, or MAY follow. At a high level, you can understand these labels as follows:

MUST Required (enforced either in the API or in Android app quality for cars)
SHOULD Recommended
MAY Optional

For details, including a discussion of MUST NOT and SHOULD NOT, visit Meaning of Must, Should & May.