The Map Object and Web Map Synergy
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In 3.x, a basemap layer, reference layer (consisting of boundaries and labels), and an operational layer are represented in the API as three individual layers. In the 4.0 API, a map is represented as a basemap and operational layer so you can easily identify and work with just the operational layers to build out workflows. You can also instantiate maps, basemaps, and layers directly from portal items because the Portal Information model is incorporated into the API.