must not
verb
- used to indicate that something is forbidden.
The problem here is actually may, not must (or must not). May can mean either optionality or regulation:
I may stop for groceries on the way home tonight.
May I have ice cream for dessert?
Your source is using it in the "optional" sense, not the "regulation" sense, so may not would also mean it's optional. Must not, on the other hand, always means that it is forbidden.
Here are uses that fit with those definitions:
Answer from Monica Cellio on Stack ExchangeI may stop for groceries on the way home tonight.
I must get gas before work tomorrow or I'll be stranded.
I must not run that red light.
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The problem here is actually may, not must (or must not). May can mean either optionality or regulation:
I may stop for groceries on the way home tonight.
May I have ice cream for dessert?
Your source is using it in the "optional" sense, not the "regulation" sense, so may not would also mean it's optional. Must not, on the other hand, always means that it is forbidden.
Here are uses that fit with those definitions:
I may stop for groceries on the way home tonight.
I must get gas before work tomorrow or I'll be stranded.
I must not run that red light.
I would recommend reading the RFC 2119. They probably tried to abbreviate the definitions:
- 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", mean that the definition is an absolute prohibition of the specification.
- SHOULD This word, or the adjective "RECOMMENDED", mean 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" mean 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", 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.)
As must expresses obligation, so must not expresses prohibition.
In answer to your question, you must not means you do not have the permission or rights to do whatever. This is equivalent to your second alternative: "you may not choose to do".
Hence, you must not does not mean your first alternative "you may choose to do or not do".
This is confusing to German learners of English in particular because the German verb müssen behaves differently from the English must, whereby "du musst nicht" does indeed mean "you may choose to do or not do".
For example:
Du musst nicht gehen
does not mean "You must not go".
It means "you don't have to go; you may choose to go or not to go."
Note: This answer is a simplistic account of just one of the functions of the complex English modal must.
I downloaded an ebook and on the first page it is written that ''you may not make this e-book publicly available in any way.''. Would the meaning be any different if they wrote you must not instead of may not?