Marco Russo and Alberto Ferrari put on their lab coats:
Fusion is a DAX optimization that reduces the number of storage engine queries when the engine detects that multiple calculations can be merged together in a single query. There are two types of fusions: vertical fusion and horizontal fusion.
Vertical fusion occurs when multiple measures – or calculations in general – need to be computed in the same filter context. For example, the following query requires the calculation of two measures: Sales Amount and Margin:
Read on to see how horizontal fusion differs and when it can be most useful.