Because there are so many questions out there, I put together this little reader, with links to most of the major testing that has been done.
Svr’s initial tests: Rend mechanics 2.6.7 - #3 by sVr-21743
WW “refreshes” stacks of Rend: [Guide] Zodiac Rend (Season 20) - #266 by Rage-13139
Ambo-Rend and HC Rend cannot stack separately, and each overwrites the other: [Guide] Zodiac Rend (Season 20) - #281 by Rage-13139
DD’s apply Rend via Ambo’s, within 9 yards of your character: Waste set bonus - #18 by Rage-13139