NST is the OST's younger sibling with a narrower job: it caches Microsoft 365 Groups. When an account belongs to 365 Groups, Outlook stores those group conversations and calendars in an NST beside the mailbox's OST, same underlying format, different content.

Whether recovery matters depends on one question: do the groups still exist in the 365 tenant? Living groups hold their real data server side and every member's Outlook simply re-caches it, making the NST as expendable as any healthy cache, nothing in it exists only there. In the common case your NST files are safely ignorable.

The exception mirrors the orphaned OST story: groups that were deleted past their own thirty day restoration window, or a whole tenant that was decommissioned, leave the NST as the only surviving copy of those group conversations. Since the format matches OST internally, the same standalone converters read it, SysTools OST Converter opens NST files and exports the group content to PST for whatever review or archiving the situation demands.

Practical order for your salvage: list the groups the account belonged to, check each against the tenant and only convert NST files backing groups that are genuinely gone. The OST meanwhile follows the usual rules covered in the mailbox recovery thread linked alongside.