The retimer context is for effects that change the length of a clip by interpolating frames from the source clip to create an inbetween output frame.
In this context, a plugin has the following mandated objects...
Any other input clips that are specified must be optional.
The 'SourceTime' parameter cannot be labelled, positioned or controlled by the plug-in in anyway, it can only have it's value read. Its value is how the source time to maps to the output time. So if the output time is '3' and the 'SourceTime' parameter returns 8.5 at this time, the resulting image should be an interpolated between source frames 8 and 9.
The pixel preferences action is constrained in this context by the following,