In an exception aggregatoin, you can use up to five reference characteristics, so that the aggregation rule will be applied in a calculation step on the combination of all these characteristics.
You want to define a calculated key figure Sales per Employee and Product and Year.
Variant 1 without MultiDimenstional Aggregation
Variant 2 with MultiDimenstional Aggregation
You can define a single calculated key figure that contains all three characteristics as reference characteristics.
This serves to simplify modeling. Thanks to all exception aggregation rules being performed on single records, without the need to calculate the substeps, performance is faster.
With multidimensional exception aggregation, it is also possible to create new analyses, to calculate average values for the combination of characteristics for example.
Person  Product  Sales in EURO  Average Sales in EURO 

Tom O'Connor  A  80   
B  40    
Total  120  60  
Max Wall  C  50   
D  70    
E  90    
Total  210  70  
Total  330 

AVG (Sales) by Product, Personcalculates on the individual values for sales of all products by all persons. This produces the following result: 330/5 = 66
AVG( AVG (Sales) by Product) by Personcalculates on the average sales per person. With this variant, the averages by products for each employee are formed first: 120/2 = 60 and 210/3 = 70. The result of the first calculation step is then used to form the average by the employees: 60+70 = 130. This produces the following result: 130/2 = 65