An Import model is a semantic model that loads data into the VertiPaq engine. The VertiPaq engine is a columnar database engine that stores data in memory and compresses it to reduce the memory footprint.
VertiPaq is the storage engine that provides the best performance for semantic models. However, it is not the only storage engine available for semantic models. A semantic model can also use the DirectQuery mode to retrieve data from the underlying data source.