To state the obvious, an official diagnosis means documentation. That is, printed manuscript of the Dx including the name/title of diagnostician, date, your name, etc.
Any diagnostic is also a legal document. So is a marriage certificate, deed, etc. No such thing as a 'verbal agreement' in any diagnostic.
For instance, should you ask for, say, an accommodation at your university (or whatever), diagnostic paperwork is required.
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The ones who say “You can’t” and “You won’t” are probably the ones scared that you will. - Unknown