If anyone is curious about what the books say, US Air Traffic Controllers use phraseology prescribed by FAA General Order 7110.65.
Also known as the 7110 or the point sixty-five, it includes rules that Air Traffic Controllers must follow to ensure safe and effective operations. So if you're a pilot and are wondering why you have a Hold For Release or are told "unable", you might find the reason in this document.
The Wikipedia article [0] links the PDF version as well as the online version.
