Shall, Should, and May 1 : Shall is used to denote a requirement; should is used to denote a recommendation; and may is used to denote permission, neither a requirement nor a recommendation
Department of Energy AREA SDMP, DOE Handbook, Glossary of Environment Safety and Health Terms, DOE-HDBK-1188-2006, D.C. 20585, USA 2006. http://www.hse.niordc.ir/uploads/86_24_dictionary_ 2006.pdf, (Envir Hlth and Sfty - Amer) {44}