1.3. Terminology

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]Note

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]editing-note

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.