Multivalued Parameter in SSRS

This explains user can SELECT more than one value from a parameter's available values. Such parameter are called Multivalued Parameters. A Query for dataset must meet the three specific conditions if you want to use Multivalued Parameter for the query.

• The Data source must be SQL Server, Oracle, SSAS, SAP BI NetWeaver, or Hyperion Essbase.
• The source query can not be a Stored Procedure. You can not pass an array to a Stored Procedure in SSRS 2008.
• The Query must be use an IN(@parameter) clause to specify the parameter.

For example: 

SELECT * FROM dbo.Employee WHERE Occupation IN(@Occupation)

If the parameter is used in the IN clause, on the General tab of Report Parameter properties dialog box, you can select the Allow Multiple Values check box. When the user views the report, the user can select all possible values, a single value, or two or more values of the parameter's available values.