...
In a contained database there is a boundary between a database and the instance of SQL Server and other databases.
A Non-contained database is a database that has containment set to NONE.
https://learn.microsoft.com/en-us/sql/relational-databases/security/contained-database-users-making-your-database-portable?view=sql-server-ver16 Use contained database users to authenticate SQL Server and SQL Database connections at the database level.
...