Microsoft® SQL Server™ 2000 introduces extended properties that users can define on various objects in a database. These extended properties can be used to store application-specific or site-specific information about the database objects. Because the property is stored in the database, all applications reading the property can evaluate the object in the same way. This helps enforce consistency in how data is treated by all of the programs in the system.
Each extended property has a user-defined name and value. The value of an extended property is a sql_variant that can contain up to 7500 bytes of data. Individual database objects can have multiple extended properties.
Extended properties are managed using three system stored procedures: sp_addextendedproperty, sp_updateextendedproperty, and sp_dropextendedproperty. You can read the value of an existing extended property using the system function FN_LISTEXTENDEDPROPERTY.
There is no convention or standard for defining extended properties. The database designer sets the rules specifying the property names and contents when the database is designed, and then the applications accessing the database have to be coded to follow those rules or conventions.