The MPL is a licence. It is issued once the course requirements are met and the student completes their base training.
The type rating for that particular Multi engine turbine aircraft is issued at the same time.
It used to come in a nice light blue folder.
Once you reach the requirements it can be changed to a full ATPL.
The licence holds the same privileges as a modular/integrated CPL.
It is designed specifically for entry into airline operations, but it is not limited to such.
You can add a whatever sep, mep, single pilot IR, seaplane, instructor or any other type to it you like because these are ratings.
If you meet he requirements, these ratings can be added to any EASA licence.
So, you do not have to stay with the same company for 1500 hours. You could technically leave the moment you finish your last circuit of base training. This is impractical due to the financial penalties and bonds associated with these schemes.
It is another route to the RHS of an airliner. Once there, in airline ops, there is no difference between an MPL and a CPL.