Look into the 'Prompt' section of Help. Here is a snippit:
Building prompts
A prompt is a dynamic filter that displays a question every time you refresh the data in a report. You respond to prompts by typing or selecting the value(s) you want to view before you refresh the data. Crystal Reports retrieves only the values you specified from the database and returns those values to the report.
Tip: Prompts allow multiple users view a single report but specify a different sub-set of the database information. Prompts also reduce the time it takes for the data to be retrieved from the database.
To create a prompt
Select the object you want to apply a prompt on and drag it to the Query Filters pane.
For example, if you want to allow users to specify the geographical region for the report, drag the Region field to the Query Filters pane.
The Filter Editor appears. By default, the name of the filter is the name of the filtered object.
- Select an operator from the list.
See Quick reference to query filter operators.
Note: You cannot use the following operators for prompts: Is null, Is not null, Both, and Except.
- Under Operator type, select Prompts
The Prompt text field shows the default message.
- If you want to change the default message, edit it to word a question.
For example, you might ask "Which region do you want to see data for?"
- Check Select only from list if you want your Prompt to display a list of values from which users can select value(s).
Use this option to prevent users from typing a value that might not exist in the database.
- Click OK to confirm the prompt.
The prompt appears at each report refresh.
Hope this helps....