Principal -- This principal objects requests authentication to SQL Server resources. There is various types of principals Windows level principals, SQL Server level principals, Database level principals.
Windows level principals are windows domain login and windows local login
SQL server level principals are server level login and server role
Database level principals are database user, database role and application role
When we install SQL Server “sa” login server level principal will created by default.
As discussed in earlier post public role (database principal) is created in every database by default.
Sys and Information_schema – These two schemas are appeared in users catalog of every database, they are not principals and these are required by SQL Server. We cannot drop or modified these entities.
Securables – This securable objects requests authorization to SQL Server resources. For example a table is securable. These securables has scopes like server, database and schema.
The server scope securables are Endpoint, login, server role and database.
The database scope securables are User, database role, application role, Assembly Schema etc.,
The Schema scope securables are tables, view, types etc.,