Transitions are effects that blend from one clip to another over time, eg: a wipe or a cross dissolve.
In this context, a plugin has the following mandated objects...
Any other input clips that are specified must be optional. Though it is suggested for simplicity's sake that only the two mandated clips be used.
The 'Transition' parameter cannot be labelled, positioned or controlled by the plug-in in anyway, it can only have it's value read, which will have a number returned between the value of 0 and 1. This number indicates how far through the transition the effect is, at 0 it should output all of 'SourceFrom', at 1 it should output all of 'SourceTo', in the middle some appropriate blend.
The pixel preferences action is constrained in this context by the following,