There are many kinds of services, but most of them share one common characteristic: they provide API specific to a particular type of plugin. A few examples:

• The CommandService works with Commands.
• The TextService works with TextFormats.
• The PlatformService works with Platforms.

Most services fit this pattern in one way or another. When you wish to provide a new extensibility point, you create a new type of SciJavaPlugin, and a corresponding PTService for working with it. Depending on the nature of your new plugin type, this service might be:

• A SingletonService, such as PlatformService, which manages SingletonPlugins.
• A HandlerService, such as TextService, which manages HandlerPlugins.
• A WrapperService, such as WidgetService, which manages WrapperPlugins.

It is named PTService rather than PluginTypeService or similar to avoid confusion with A) the PluginService itself, and B) any other service interface intended to define the API of a concrete service. In contrast to such services, the PTService is a more general layer in a type hierarchy intended to ease creation of services that fit its pattern.