The Variant data type is used for expressions that might return different data types, depending on the conditions. For example, the following statement can return either an integer or a string, so it returns a variant type:
IF ( [measure] > 0, 1, "N/A" )
The Variant data type cannot be used as a data type for a column in a regular table. A DAX measure and in general a DAX expression can be Variant.
Contributors: Alberto Ferrari, Marco Russo