Title: Theory / Unimportant / CS / Programming / RFC / 2119 Key words for use in RFCs to Indicate Requirement Levels

Name: SHALL
Definition: The definition is an absolute requirement of the specification.
Name2: REQUIRED
Name3: MUST

Name: SHALL NOT
Definition: The definition is an absolute prohibition of the specification.
Context: RFC
Name2: MUST NOT

Name: RECOMMENDED
Definition: There may exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understood and carefully weighed before choosing a different course.
Context: RFC
Name2: SHOULD

Name: NOT RECOMMENDED
Definition: There may exist valid reasons in particular circumstances when the particular behavior is acceptable or even useful, but the full implications should be understood and the case carefully weighed before implementing any behavior described with this label.
Context: RFC
Name2: SHOULD NOT

Name: OPTIONAL
Definition: An item is truly optional. One vendor may choose to include the item because a particular marketplace requires it or because the vendor feels that it enhances the product while another vendor may omit the same item.
Name2: MAY

Question: What's important with «MAY» options.
Answer: That, independently of whether you implement it or not; your must accept to interact with implementation making another choice
Page: https://tools.ietf.org/rfc/rfc2119.txt

Question: What's important with «MUST» or «MUST NOT» options.
Answer: You must explain why it MUST (not) be done; and what are security risks related to them
Page: https://tools.ietf.org/rfc/rfc2119.txt