What/which

[Allen165]

"As I understand it, exceptions are possible, but I don't know under what/which conditions."

"What" sounds more natural to me, but I'm not sure if it's truly better than "which."

Thanks.

[euncu]

***neither a teacher nor a native-speaker***

"What" makes it an open set, while "which" makes it a closed one.

what conditions: Any condition, no number can be given.
which conditons: aforementioned conditions, of limited number.