Rules are database objects that specify the acceptable values that can be inserted into a particular column. Rules are bound to a column or to a user-defined datatype. A column or user-defined datatype can have only one rule associated with it. (Note that using constraints is the preferred way of restricting column data because multiple constraints can be defined on a column or on multiple columns.)
For example, in a table with a column called phone_number, you could define a rule that specifies that the column accepts only 10-digit entries. For details on creating rules, see the CREATE RULE statement in the Microsoft SQL Server Transact-SQL Reference.