An update is a cumulative set of bug fixes applied to an existing installation of MATLAB. Information about limitations and bugs fixed in MATLAB Updates are described in the release notes linked from the download page.
Bug reports also indicate in which Update release a specific bug was fixed and redirect to the download page.
Either you have access to the pre-release, in which case you should have received a link to the pre-release notes in the email telling you you can download it, or you don't, in which case we're not allowed to share these with you.
Pre-releases come with an NDA (at least for me), meaning we're not allowed to share the details of what's in them.
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