RFC 6919
Everyone who has ever worked with RFCs will appreciate this:
RFC 6919 - Further Key Words for Use in RFCs to Indicate Requirements Levels
RFC 2119 defines a standard set of key words for describing
requirements of a specification. Many IETF documents have found that
these words cannot accurately capture the nuanced requirements of
their specification. This document defines additional key words that
can be used to address alternative requirements scenarios. Authors
who follow these guidelines should incorporate this phrase near the
beginning of their document:
The key words "MUST (BUT WE KNOW YOU WON'T)", "SHOULD CONSIDER",
"REALLY SHOULD NOT", "OUGHT TO", "WOULD PROBABLY", "MAY WISH TO",
"COULD", "POSSIBLE", and "MIGHT" in this document are to be
interpreted as described in RFC 6919.
Quotes:
The phrase "SHOULD CONSIDER" indicates that the authors of the
specification think that implementations should do something, but
they're not sure quite what.
The phrase "REALLY SHOULD NOT" is used to indicate dangerous
behaviors that some important vendor still does and therefore we were
unable to make MUST NOT.
The phrase "MAY WISH TO" indicates a behavior that might seem
appealing to some people, but which is regarded as ridiculous or
unnecessary by others. This phrase is frequently used to avoid
further delay in approval of a document.
The phrase "POSSIBLE" describes what some of the working group
members thought of as an edge case that will never happen, but in
practice allows the protocol to work at the most fundamental level.
RFC 6919 - Further Key Words for Use in RFCs to Indicate Requirements Levels
RFC 2119 defines a standard set of key words for describing
requirements of a specification. Many IETF documents have found that
these words cannot accurately capture the nuanced requirements of
their specification. This document defines additional key words that
can be used to address alternative requirements scenarios. Authors
who follow these guidelines should incorporate this phrase near the
beginning of their document:
The key words "MUST (BUT WE KNOW YOU WON'T)", "SHOULD CONSIDER",
"REALLY SHOULD NOT", "OUGHT TO", "WOULD PROBABLY", "MAY WISH TO",
"COULD", "POSSIBLE", and "MIGHT" in this document are to be
interpreted as described in RFC 6919.
Quotes:
The phrase "SHOULD CONSIDER" indicates that the authors of the
specification think that implementations should do something, but
they're not sure quite what.
The phrase "REALLY SHOULD NOT" is used to indicate dangerous
behaviors that some important vendor still does and therefore we were
unable to make MUST NOT.
The phrase "MAY WISH TO" indicates a behavior that might seem
appealing to some people, but which is regarded as ridiculous or
unnecessary by others. This phrase is frequently used to avoid
further delay in approval of a document.
The phrase "POSSIBLE" describes what some of the working group
members thought of as an edge case that will never happen, but in
practice allows the protocol to work at the most fundamental level.