The retimer and transition context both mandate a parameter be declared, the double params 'SourceTime' and 'Transition'. The purpose of these parameters is for the host to communicate with the plug-in, they are not meant to be treated as normal parameters, exposed on the user plug-in's user interface.
For example, the purpose of a transition effect is to dissolve in some interesting way between two separate clips, under control of the host application. Typically this is done on systems that edit. The mandated 'Transition' double pseudo-parameter is not a normal one exposed on the plug-in UI, rather it is the way the host indicates how far through the transition the effect is. For example, think about two clips on a time line based editor with a transition between them, the host would set the value value of the 'Transition' parameter implicitly by how far the frame being rendered is from the start of the transition, something along the lines of...
Transition = (currrentFrame - startOfTransition)/lengthOfTransition;
This means that the host is completely responsible for any user interface for that parameter, either implicit (as in the above editing example) or explicit (with a curve).
Similarly with the 'SourceTime' double parameter in the retimer context. It is up to the host to provide a UI for this, either implicitly (say by stretching a clip's length on the time line) or via an explicit curve. Note that the host is not limitted to using a UI that exposes the 'SourceTime' as a curve, alternately it could present a 'speed' parameter, and integrate that to derive a value for 'SourceTime'.