Essence is the main MMGIS frontend — the thing users actually look at. It lives under src/essence/ and is bundled by the custom Webpack pipeline described in Build and bundling. It is large enough that a single page would not be useful: there is a core runtime that owns a global map state and swaps between map engines, a tool registry where most of the user-facing functionality lives, a wide surface of non-map UI chrome, and a separate public JavaScript API for embedders. Each of those is its own concern with its own conventions, so each gets its own page.

The thing to know up front is that Essence is not an idiomatic React app. There is React 16 in the tree (the panels, much of the chrome), but the map layers, the global state object L_, and most tool internals are imperative ES modules — many descended from a much earlier jQuery/Leaflet codebase. The newer code (the mmgisAPI bus, some of the Tools) is moving toward a mitt-based pub/sub model, but the old shape is still load-bearing. Read the core runtime page first; it explains the global object every other piece reaches into.

In this component

• Core runtime and map engines — the boot sequence, the global L_ map state, the Basics/ controllers (Map_, Globe_, Viewer_, PanelManager_, UserInterface_, ToolController_, TimeControl_, ComponentController_), and how 2D ↔ 3D ↔ image-viewer switching works. This is the spine the rest plugs into.

• Mapping tools — the contract every tool follows, how the ToolController loads them, and a tour of what ships in src/essence/Tools/ (Draw, Measure, Identifier, Layers, Isochrone, Animation, Curtain, Viewshed, Chemistry, Sites, Info, Legend, Shade, Kinds). Also covers the plugin-drop convention used to add custom tools.

• Ancillary UI and chrome — everything around the map. Login, search, context menu, modals, scale bar, coordinate readout, help, query-by-URL, attributions, plus the Helpers/ utilities and the LandingPage/. Mostly imperative ES modules with paired CSS, mounted by the layout machinery in Basics/.

• mmgisAPI (embed/extension surface) — the stable JavaScript API exposed on window.mmgisAPI for iframe embedders and plugin authors. Two surfaces in one file: a JSDoc'd public method set and a newer mitt-based event/request bus used by tools to talk to each other and to the core without import-coupling.

The boundary that matters most: tool code goes in Tools/, persistent UI chrome goes in Ancillary/, and any code that coordinates tools, layers, panels, and engines belongs in Basics/ (core runtime). When in doubt, look at where similar code already lives.