ArcGIS API for JavaScript
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Online Content Help - Migrating map tiling schemes
What is ArcGIS Online Content? ArcGIS Online Content includes online maps and tasks published and hosted by Esri that you can use as your basemap framework for deploying GIS maps and applications to your users both inside and outside your organization.
37329 - Make a cache using World Mercator Auxiliary Sphere (wkid 102100) for use with Bing Maps and ArcGIS Online services
Summary When caching using the 'Bing Maps/Google Maps' tiling scheme in ArcCatalog or ArcGIS Manager, the cache is based on Web Mercator with an ID of 102113. If the underlying data used in the cache has a datum that is not in WGS 1984, the resulting cache will not overlay correctly with Bing Maps or ArcGIS Online services that are already in the 'Bing Maps/Google Maps' scheme.
I'm not aware of any issue with doing this but your tiled layer would need to be cached with transparency turned on for you to see your dynamic layer below. You'd also have to ensure that the image format you select when caching supports transparency (PNG for example).