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FIGURE A.3 Downward propagation—if a requirement attribute value is set at
a higher level, it is automatically applied to all derived lower level requirements.

A.4 Unique Needs for a Product Line RDB
Product lines can impose additional burdens on an RDB (see
Chapter 6). Just a few issues will be described here. As always, it is
best to plan for the RDB implementation through process definition
and an artifact model (see Chapter 2).

Multidimensional Support
A product line consists of several products with some shared features
and some divergent features. This means that the relationship between
product release, product definition, and product line definition is
three-dimensional. In Figure A.4 you see that products in a product
line may or may not implement a requirement. Furthermore, even if a
product is destined to implement the requirement, it may take several
releases before it does. In order to support a product line, then, it must
be possible to support the generation of three-dimensional structures.

Generation of Product Maps
A product map shows, for any given product line, which product
features will be in a specific product (Figure A.5). As many
requirements may be associated with a product, it is important that
when maps containing product reports are created, they be filterable
so that the generated map is understandable; e.g., only has
requirements at the same level shown.