SQL DB V12 supports diagnostic connections (DAC) to the SQL DB V12 databases similar to the SQL Standalone. Using this an admin can connect to the database to troubleshoot the issues when he can’t connect to the database (for example concurrent session limit reached).
The port to connect is 1434. However it appears one can’t connect to the master database.
If one offending session took a lock on an object where rest all the concurrent sessions waiting on that session to finish we end up in a situation where admin can’t connect to the database to recover it. This is when one can open DAC connection to the database and kill the offending session.