Conventions

In this document, the keywords "MUST", "REQUIRED", "SHALL", "MUST NOT", "SHALL NOT", "SHOULD", "RECOMMENDED", "SHOULD NOT", "NOT RECOMMENDED", "MAY" and "OPTIONAL" are used to specify the requirement levels in accordance with RFC 2119.

keyword	other terms	description
MUST	REQUIRED, SHALL	This is an absolute requirement.
MUST NOT	SHALL NOT	This is an absolute prohibition of the specification.
SHOULD	RECOMMENDED	This means you can choose to disregard a requirement if you have a valid reason of that choice.
SHOULD NOT	NOT RECOMMENDED	This means that an item might be acceptable, but you should fully understand the consequences.
MAY	OPTIONAL	This means that an item is truly optional.