A filter effect is the ordinary way most effects are used with a single input. They allow track or layer based hosts that cannot present extra input to use an effect.
In this context, a plugin has the following mandated objects...
Other input clips may be described, which must all be optional. However there is no way to guarantee that all hosts will be able to wire in such clips, so it is suggested that in cases where effects can take single or multiple inputs, they expose themselves in the filter context with a single input and the general context with multiple inputs.
The pixel preferences action is constrained in this context by the following,