An Import model is a Tabular 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 Tabular models. However, it is not the only storage engine available for Tabular models. A Tabular model can also use the DirectQuery mode to retrieve data from the underlying data source.