Grade options are specified with null values (or in the case of invokers, fluid.notImplemented may be used), optionally with a comment describing the role of the option, or who is expected to provide the value. These grade options document expectations of code that will work with components of the grade.
The kettle.middleware grade documents the middleware invoker interface, a single invoker called "handle", using fluid.notImplemented:
// The base middleware grade defines a function accepting the request object and returning a promise
A marker grade is a grade, typically with no implementation, that is used to annotate a component as having a particular role or position in the component tree. The marker grade may then be used, for example, to locate the annotated component in IoC expressions.
The Nexus Co-Occurrence Engine uses a marker grade gpii.nexus.componentRootHolder to indicate the parent of the Nexus "component root":
In this case, the marker grade provides flexibility in the component structure that the Co-Occurrence Engine is participating in. The gpii.nexus.componentRootHolder need only be findable by the Infusion IoC system. For example, the component root holder could be a sibling, a parent, or a grandparent of the Co-Occurrence Engine.
The component options being distributed are specified at a distance from the component(s) that will be affected. This quality may make it hard to see the source of all of the configuration options that a component will receive.
Kettle configurations of the GPII use distributeOptions to inject configuration and sub-components into parts of the system