What is it?
Use SUMBY to aggregate a measure up while eliminating dimensions. SUMBY is very powerful for summary tables, where detailed source tables have dimensions for workings not required in a summary view and where you wish to reduce the summary dimensions to a consistent standard. You can SUMBY one or more dimensions.
How to write it?
Fx layout: = SUMBY(Table.Measure, Dimension1.LevelA, Dimension2.LevelB, etc.)
Example = SUMBY(Product plan.Volume, All Sales reps, All Products)
Will result in the volume of products measure from the Product plan table being displayed in the resulting measure without the Sales reps and Products dimensions (assuming those dimensions are not already present/utilized elsewhere in that table).
*Related measure: Arithmix Functions: SUM (aggregating measures to higher hierarchy levels in dimensions)