Cost Profile vs Startup String

Database

Cost Profile vs Startup String

A business user is running reports that are using OLAP functions in the queries.

The performance analyst has suggested to use the below diagnostic command , in all the sessions for the user queries  to improve performance:

DIAGNOSTIC HASHSTATSPART ON FOR SESSION;

This is a part of Cost profile, so the user is requested to modify his profile to the new cost profile.

I want to know how does cost profile include the diagnostic command for the user?

Is it equivalent to associate the user's sessions with a startup string that executes the diagnostic command?

Whats the difference between assigning a cost profile and startup string?