Analysis Services has 3 instance install modes. Multidimensional is for the "legacy" Analysis Services cubes. Tabular is for a standalone tabular server hosting persisted tabular models, or temporary models created by Visual Studio. Sharepoint mode is for integration with PowerPivot functionality in Sharepoint.
There are some key features which may force you to select both the Tabular and PowerPivot modes for different functional requirements.
Understanding Analysis Services Deployment Modes - Analysis Services and PowerPivot Team Blog - Site Home - MSDN Blogs: "Obviously, this way of connecting to a PowerPivot database is very different from connecting to a Tabular database, but the differences between Tabular and SharePoint modes go even deeper than that because the databases themselves are very different in nature. Tabular databases
are permanent databases. PowerPivot databases, on the other hand, are temporary. They can be deleted from the SSAS server and reloaded from their workbooks at any time."
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