In my knowledge, mono db is only for the internal uses and able to access with internal Splunk SPL.
Is there any official documentation about this?
Thank you in advance.
The Splunk KVstore feature is implemented using mongoDB. This is mostly transparent, however.
The KVstore is not just for internal use. Any Splunk app/search can use the KVstore.
The KVstore is documented in the Admin Manual at https://docs.splunk.com/Documentation/Splunk/8.2.5/Admin/AboutKVstore