Understanding the BI semantic model architecture

Analysis Services is a foundational component of the Microsoft BI. It has the ability to host analytical models that can be used with tools, such as Excel through PivotTables. The model can either be multidimensional OLAP models or in-memory models called tabular models. This model is called the BI Semantic Model (BISM).

The BI Semantic Model is a new model that includes both the classic multidimensional model or OLAP model and the newer tabular model.

The OLAP model is a true multidimensional model that allows the developers to include very complex business logic into the model. The tabular model is a relational model that is easier to build, but currently does not have the same advanced possibilities that you have in the multidimensional model. Both models are handled by the server instance, but not at the same time. You have to decide what model you want to use when installing the instance. There is no way of changing this except reinstalling the instance.