The key words must, must not, required, shall, shall not,
should, should not, recommend, may, and optional in this document
are to be interpreted as described in RFC 2119:
http://www.ietf.org/rfc/rfc2119.txt
-
must
-
This word, or the terms required or shall, mean that the
definition is an absolute requirement of the specification.
-
must not
-
This phrase, or the phrase shall not, means that the
definition is an absolute prohibition of the specification.
-
should
-
This word, or the adjective recommended, means 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.
-
should not
-
This phrase, or the phrase not recommended, means 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.
-
may
-
This word, or the adjective optional, means 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).
The additional terms can and cannot are to be interpreted as follows:
-
can
-
This word means that the particular behavior described is a valid
choice for an application, and is never used to refer to implementation
behavior.
-
cannot
-
This word means that the particular behavior described is not
achievable by an application. For example, an entry point does not exist, or
shader code is not capable of expressing an operation.
![[Note]](images/icons/note.png) | Note |
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There is an important distinction between cannot and must not, as used
in this Specification. Cannot means something the application literally is
unable to express or accomplish through the API, while must not means
something that the application is capable of expressing through the API, but
that the consequences of doing so are undefined and potentially
unrecoverable for the implementation. |
![[Note]](images/icons/note.png) | editing-note |
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TODO (Jon) - We might need to augment the RFC 2119 definition of must not
to include some of the previous note, since at present it is defined solely
in terms of implementation behavior. See Gitlab issue #9. |