"which is" 可以用于 定语从句或 选择疑问句。

定语从句

"which" 作为关系代词，引导一个定语从句，用来修饰先行词。例如：

The book, which I borrowed from the library, was very interesting.

The hotel, which we stayed at last year, had a beautiful view of the ocean.

选择疑问句

"which is" 也可以用于构成选择疑问句，通常与 "or" 连用，例如：

Which is my seat？

根据具体的语境和句子结构，"which is" 可以明确地用于定语从句或选择疑问句中。希望这些解释对你有所帮助。