What is the return value of the $group stage in MongoDB?

The $group stage in MongoDB is used to aggregate documents based on a specified key, which can be a field or set of fields in the documents being processed. When you use the $group stage, the returned output consists of one document for each unique value of the specified group key. This is essential for summarizing data and generating aggregate statistics.

For example, if you group by a field such as "category," the output will include a separate document for each unique category present in the original dataset. Each of these documents typically contains the group key and aggregate values calculated from the documents that belong to that group, such as sums or averages.

This behavior of producing one document per unique group key allows developers to efficiently analyze and report on multidimensional data, facilitating decision-making based on aggregated results.