I am trying to develop some applications using Xyce. Are there some documents/examples of using its API? Thanks

Unfortunately, there is not just one API, there are several (though a potential future project may be to consolidate). As of now, the API docs you'd need depend on the job and language. There's a trivial API to just call Xyce about the same way one would from the command line - is that what you need? Sorry for the late reply. Somehow missed this message until today.