For both import and DirectQuery data models, it's possible to create additional model columns via DAX functions.
In almost every scenario, a superior alternative to DAX calculated columns is available such as via M expressions or the SQL database views that the model accesses. DAX calculated columns are not compressed like normal columns of import mode datasets and can lead to inefficient queries for DirectQuery models. Additionally, the presence of DAX calculated columns (and DAX calculated tables for import models) adds complexity to Power BI datasets.
Greater analysis of DAX calculated columns is included in Chapter 11, Enhancing and Optimizing Existing Power BI Solutions.