A customer was looking for information about the file
%LOCALAPPDATA%\Microsoft\.
All they could find is
this KB article.
The
NTDiskQuotaSidCache.ndx
file is used by the Disk Quota control panel.
It's a cache of SIDs and names,
to improve performance of mapping SIDs to display names.
You can delete it, and it will be regenerated the next
time you run the Disk Quota control panel.
The customer never explained what they needed the information for, but they did thank us. (Or at least their liaison thanked us on the customer's behalf.)