>它定义的数据结构是什么？
是否适应其他语言？
>它在哪里用于AngularJS？该API是否仅存在于select元素？

From the docs：

ngOptions – comprehension_expression – in one of the following forms:

for array data sources:

• label for value in array
• select as label for value in array
• label group by group for value in array
• select as label group by group for value in array track by trackexpr

for object data sources:

• label for (key,value) in object
• select as label for (key,value) in object
• label group by group for (key,value) in object
• select as label group by group for (key,value) in object