Dwain C has a nice article covering usage, I've put a link in an earlier post in this thread.
Blimey. I've learned so much from your website over the years. Nice - and a privilege - to see the tables turned.
For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden