Tablix enables the separation of data groupings and layout header areas. A stepped layout can help with horizontal spacing, because nested groups can then share the same horizontal space, which was previously not possible in a matrix layout. This provides very powerful layout capabilities, and is one among several new features introduced with Tablix.
A webcast about Report Authoring in Reporting Services 2008 by my esteemed colleague Carolyn Chau shows how to design a table or matrix with a stepped group layout by simply selecting an option in the table and matrix wizard.
You can accomplish the same effect by manually converting from a regular blocked to a stepped layout. For the example shown below:
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