At the Adobe Support Community, in Lightroom Classic, Under Discussion, right near the top, notice the document link that is ALWAYS provided when updates/upgrades are released. Here:
I have encircled the link in the screenshot above, go in the actual web page, and click on that link
Notice the link within that document that I encircled, on the actual web page, click on it
That brings up a document Adobe ALWAYS posts at updates and upgrades. Several articles may be included. Especially at upgrades. Notice that I circled two, you will want/need to bring up and read both. For now, for the subject at hand, on the actual web page, click on that one circled in red, the one about Masking.