Codes - Official, must do. eg. ASME Section VIII for pressure vessel.
Standards - We do things this way so life is easy. eg. ANSI pressure ratings. Also, companies may have their own standards, to make sure work is similar.
Recommended Practices - I suggest you do it this way because it has worked for me in the past. eg. ASME RP's. You don't have to do it this way, it is just recommended.
I am sure there are other definitions, and other ways people use these words.
"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
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