When the cached values of these books should be re-read from their sources, on open.
Remarks
userSet asks the user on open, always refreshes without asking, never uses the cache.
The mode governs the books as a SET — no single book carries one — but it is read and written
on the workbook (IWorkbook.getUpdateLinks), where Excel's object model and the file both
keep it.
When the cached values of these books should be re-read from their sources, on open.