I don't believe there is a specific 'plugin' reference, I've figured out most everything by trial/error and reading a lot of programming books to get going.
First reference to use is Robert Spark's macro reference (if that is what you're referring to). Get it from the signature line of his posts. It isn't specific to plugininterface/plugins, but you'll find most all the 'macro' functions are available in the plugin side as well, and the descriptions/usage carry over well.
Second, open the sample plugin/template file UCCNC provides in VisualStudio. Use object browser or start typing and intellisense will pop-up the available functions with enough info to figure it out. The available functions mirror Robert's manual good enough.
Third, ask here and we'll all try to help
There is also the UC100 API interface, which has more motion control related functions, depending on what you're trying to do and accomplish.
regards
Eric