Microsoft Power BI Complete Reference

Fact column data types

It's essential that the numeric columns of fact tables are assigned to the appropriate data types. All integer columns, such as Order Quantity, should be stored as a whole number data type, and decimal numbers will be stored as either fixed decimal numbers or as decimal numbers. If four decimal places is sufficient precision, a Fixed decimal number type should be used to avoid rounding errors and the additional storage and performance costs of the decimal number type.

In the following screenshot, the Freight column is stored as a Fixed decimal number type and, thus, despite a format of six decimal places, only four significant digits are displayed to the right of the decimal place: 

Fixed decimal number data type