What does "Update and Reconcile" update?

[Donald Pellegrino]

When importing transactions, the "Generic import transaction matcher"
dialog provides "A," "U+R," and "R" columns. These columns each contain
check-marks but are mutually exclusive in their selection. It is not clear
to me which fields of the transaction will be changed, and in which ways,
when "U+R" (Update and Reconcile) is selected instead of just "R"
(Reconcile).

For reference, I found an old post discussing the topic at
https://lists.gnucash.org/pipermail/gnucash-user/2011-November/042039.html.
However, I was unable to find a definitive documentation of the full
process. As with the author of that old post, I am nervous to lose the
Description content that I have entered.