Explanation/Reference:
Note:
To set a filter on the dataset
1. Open a report in Design view.
2. Right-click a dataset in the Report Data pane and then click Dataset Properties. The Dataset Properties dialog box opens.
3. Click Filters. This displays the current list of filter equations. By default, the list is empty.
4. Click Add. A new blank filter equation appears.
5. In Expression, type or select the expression for the field to filter. To edit the expression, click the expression (fx) button.
Box 1: Here we use the Fields expression.
6. From the drop-down box, select the data type that matches the type of data in the expression you created in step 5.
7. In the Operatorbox, select the operator that you want the filter to use to compare the values in the Expressionbox and the Valuebox. The operator you choose determines the number of values that are used from the next step.
Box 2: we test for equality.
8. In the Valuebox, type the expression or value against which you want the filter to evaluate the value in Expression.
Box 3: we compare to the value of the Parameter named Site.
9. Click OK.
Reference: How to: Add a Filter (Reporting Services)