Konformität

http://www.ietf.org/rfc/rfc2119.txt

1. MUST This word, or the terms "REQUIRED" or "SHALL", mean that the

  definition is an absolute requirement of the specification.

2. MUST NOT This phrase, or the phrase "SHALL NOT", mean that the

  definition is an absolute prohibition of the specification.

3. SHOULD This word, or the adjective "RECOMMENDED", mean that 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.

4. SHOULD NOT This phrase, or the phrase "NOT RECOMMENDED" mean that

  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.

5. MAY This word, or the adjective "OPTIONAL", mean that 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.
  An implementation which does not include a particular option MUST be
  prepared to interoperate with another implementation which does
  include the option, though perhaps with reduced functionality. In the
  same vein an implementation which does include a particular option
  MUST be prepared to interoperate with another implementation which
  does not include the option (except, of course, for the feature the
  option provides.)

6. Guidance in the use of these Imperatives

  Imperatives of the type defined in this memo must be used with care
  and sparingly.  In particular, they MUST only be used where it is
  actually required for interoperation or to limit behavior which has
  potential for causing harm (e.g., limiting retransmisssions)  For
  example, they must not be used to try to impose a particular method
  on implementors where the method is not required for
  interoperability.