EDMC Overlay is an EDMC Plugin for *other* EDMC Plugins and other software. It provides the means for direct in-game visual updates either from an EDMC Plugin or directly from another tool via a simple JSON API.
https://www.youtube.com/watch?v=RTZwa8oUpAc
EDMCOverlay is the backend that powers visual display for EDMC HITS and The NULL EDMC BGS Tracker Plugin https://www.youtube.com/watch?v=peAReThq99s