In Eclipse 2.1, the notion of a primary feature was used to customize the product-branding aspects of a product. This mechanism is still supported in Eclipse 3.0, but is effectively deprecated. Newly developed products should use the products extension point to define a product.
The remainder of this discussion describes the legacy product definition using primary features.
When the Eclipse platform is started, exactly one feature can control the overall "personality" or "branding" of the platform, including the splash screen, window images, about box, welcome page, and other customizable aspects of the platform. This feature is called the product's primary feature.
Let's look again at the description of the platform feature from Eclipse 2.1:
<?xml version="1.0" encoding="UTF-8"?>
<feature
id="org.eclipse.platform"
label="%featureName"
version="2.1.0"
provider-name="%providerName"
plugin=""
image="eclipse_update_120.jpg"
primary="true"
application="org.eclipse.ui.ide.workbench">
...
The platform feature has been designated as a primary feature. Although it's possible to designate many features as primary features in their feature.xml files, only one primary feature gets control when the platform is launched. This is controlled by setting the eclipse.product property in the product's config.ini file underneath the eclipse/configuration directory. If there are multiple eligible primary features, the -product command line option for eclipse.exe overrides the choice made in config.ini.