In the MongoDB documentation https://docs.mongodb.com/manual/reference/program/mongod/ i find the following definition.
New in version 3.0.0.
When you start mongod with
--wiredTigerDirectoryForIndexes , mongod stores indexes and collections in separate subdirectories under the data (i.e.
--dbpath ) directory. Specifically, mongod stores the indexes in a subdirectory named
index and the collection data in a subdirectory named
Mounting the different directories collection and index on different disks isn’t a task performed by the wiredTigerDirectoryForIndexes option.
If the item nr. 6 were a mistake then correcting my answer as PASS and upgrading my final rate please.