Parameters provide you with a way to use a different value each time you use a record filter. A parameter acts as a placeholder for a value that needs to be defined each time the report is run. When you are ready to generate a report that contains a parameter in an expression, enter a value for each parameter. Those actual values are then used as part of the report criteria for retrieving records.
To identify an expression as a parameter, add two @ symbols in the Value/Parameter cell.
To use a parameters
If a space is required in a parameter name, use the underscore ( _ ) character to represent a space. Example: @@Todays_Date
Important: The value entered in the Value / Parameter cell is used only to preview your parameters in the report. This value is not saved.
In the future, you can run this report from the General tab and enter the parameter values from there.
For more information, "Running Reports".