If I use the Export menu in Preview it gives me an option to make various versions of my PDF including PDF/A. What PDF/A is this, 1b, 2b, 2u, other?
I have been unable to find any documentation by Apple about this addition to Preview (or in PDFKit), since it was introduced.
However, a quick test of saving a PDF/A file from Preview and 'flight-checking' it in Acrobat reveals that it does not comply entirely with any PDF/A standard. PDF/A-2b comes closest, with some minor metadata infractions.
It's possible that these problems are specific to the file, and that other PDFs might have compliant metadata.
As with Apple's PDF/X implementation, I would not rely on it. For example, you can check the box to Create a PDF/A document, and also to encrypt it, which is against the spec.
(In fairness, you get an encrypted PDF of blank pages.)
Warning: Preview does not open PDF/A files as read-only, and any modification to the file will be auto-saved, even if contrary to the spec, while maintaining the flag for PDF/A.
In short: do not use Preview to either view or create PDF/A documents.