There seem to be three terms used by experts in the field I'd use the two hyphens. However, the other two seem to be more commonly used in that context.
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Except non is not an english word, it is a prefix of latin origin
Which is why american style manuals will always ask you to merge it with the subsequent word, without a hyphen
In expressions in which figures and abbreviations (or symbols. Leaving non stranded doesn't work either as it is a bound morpheme, a prefix not a word (in english)
