I understood the associated enpoint return one mbid for each mbid send ? How is this relation made ?
The endpoint will 1 mbid for each mbid sent and some metadata like artist name, recording name etc.
The mbid returned will be different from the one sent in case of redirects. Say there were 2 recordings in MB but at some point it was discovered that one is a duplicate of the second so both were merged into one. So the mbid which was merged will then have a redirect to the other. In all other cases, the same mbid will returned back.
It looks like this is intented to create a playlist based on a list of mbids but not from only one mbid. Is that correct ?
You can create a list of just 1 mbid and it will create a playlist of 1 mbid in that case :).