If you want to filter out the data your report returns such as only people whose last name starts with letter A-L, you'll want to use the SelectionFormula for filtering out the data at runtime.   The formula editor allows you to create design-time filters.  Here's the link to a tutorial that has the sample code for using the Selection Formula to do this filtering out of data:

http://www.businessobjects.com/products/dev_zone/granville_beta/GranvilleBetaOnlineHelp/html/crtsktutorialscrvselectionformulas.htm

The selection formula tutorial uses the API, and therefore is intended to create run-time filters.