Credentials (Database Engine)
A credential is a record that contains the authentication information (credentials) required to connect to a resource outside SQL Server. This information is used internally by SQL Server. Most credentials contain a Windows user name and password.
The information stored in a credential enables a user who has connected to SQL Server by way of SQL Server Authentication to access resources outside the server instance. When the external resource is Windows, the user is authenticated as the Windows user specified in the credential. A single credential can be mapped to multiple SQL Server logins. However, a SQL Server login can be mapped to only one credential.
System credentials are created automatically and are associated with specific endpoints. Names for system credentials start with two hash signs (##).
For more information about credentials, see the sys.credentials catalog view.