In general, each of the extension points defined in the UI follow a
simple pattern. The extension point is defined within plugin.xml
for org.eclipse.ui. Then extensions are defined for the
extension point. These may exist in the same or other plugin.xml
files. The top level XML element for any extension may contain one
or more extension declarations. For example, an org.eclipse.ui.propertyPages
extension may contain several instances of page elements. Although
making one extension per subelement is not technically incorrect, we recommend
grouping for consistent style. It can also improve workbench startup time
because there will be fewer extensions to process.