'bsondump --pretty --type=debug' use case and output explanation
The bsondump utility converts BSON data (which is binary) into human-readable formats. Common usage would be bsondump --pretty to inspect data formatted as MongoDB Extended JSON. This can be useful for previewing data contained in BSON files as created by mongodump, replica set rollbacks, and other features (eg. auditing in MongoDB Enterprise as per your example). 

The --type=debug output is generally only relevant for a developer who wants to see an annotated view of the BSON (as per the BSON spec).


  What's --- new object ---? It seems not to be a JSON document. 


This indicates the start of a new BSON object, which would be represented as an opening curly brace ( { ) in JSON. You will see this at the top level for each new document and for every embedded document/array.


  What's type and the number? Is size characters or other measurement?


This indicates the BSON type and size (in bytes) of that object including nested fields/objects.

Potential use cases for confirming underlying data types might be checking for consistency (eg. a given field has the expected type) or  confirming underlying data types in order to do a query using the  $type operator in MongoDB.

You can perform a similar size calculation for a document/object in the mongo shell using Object.bsonsize(object).