What is "JavaScript with Scope"

[Chang Su]

In the following link 

https://docs.mongodb.com/manual/reference/bson-types/

It mentions JavaScript with Scope is a possible data type in documents.

My questions are:

(1) What is a JaveScript with scope?

(2) Is it some kind of "Internal" data types in MongoDB

   By “internal”, I mean it cannot be used by users.

    I didn't find any more info about this type, except it is mentioned in above link 

(3) In mongo c driver, I found Struct bson_value_t

    http://mongoc.org/libbson/1.0.0/bson_value_t.html

    what is the "scope_data" buffer ?