Thanks for making reference to the 2018 US Dahua catalog, I hadn't seen it before:
http://us.dahuasecurity.com/wp-content/uploads/2018-Dahua-Technology-Full-Product-Catalog-web.pdf
I am glad that it explains what they're doing with the term Starlight, because IMO they're using it with US camera models that do not perform as well as International Starlight camera models.
Dahua US:
Nice to see that the 2MP International Starlight models that most folks here have fall into the Dahua US "enhanced Starlight Technology for professional applications" sub-designation.
Is this what they mean by Starlight:Best?
Here's a professional review of a Dahua US 5MP "Starlight" dome. Dahua US' description above puts it in their "cost effective, basic version of Starlight" sub-designation.
Is this what they mean by Starlight:Good? Ugh
We (IPVM) tested the 5MP varifocal stralight model (same series, same sensor) and it tested quite poorly. Unlocked here -
Dahua 5MP Starlight Camera Tested (N52BM3Z)
Back to your original question... with Andy
@EMPIRETECANDY having mentioned that he won't be able to buy the 5231R-Z for much longer, my *guess* is that it's being EOL'ed... and if that's right, it wouldn't make sense to introduce an EOL model to the US market. Maybe the 5231R-ZE will make it over here one day.
Regarding Hikvision, someone has already posted a 5231R-Z compared to a Hik Ultra-Low Light model review somewhere on the Hik forum here, IIRC.