The word 'may', in the sense that good old Webster uses first, has been clarified by RFC 2119, "Key words for use in RFCs to Indicate Requirement Levels", along with 'should' and 'must' to ensure that specifications for interoperability. RFC 2119 refers to these words in capitals, and also states the meanings of 'should not' and 'must not'.

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.)
The RFC does not define 'may not': the meaning would be mostly similar to that of 'may'; namely, that it is completly optional, and not failing to remove surplus negatives is never a bad thing.