Rules in SQL Server 2005
From MSDN


  *condition_expression* includes one variable. The at sign (@) precedes
  each local variable. The expression refers to the value entered with
  the UPDATE or INSERT statement. Any name or symbol can be used to
  represent the value when creating the rule, but the first character
  must be the at sign (@).


http://msdn.microsoft.com/en-us/library/ms188064.aspx