Nested Iterators

[chahineatallah]

hello, am having hard time understanding nested iterators

I created one small table with quantities but still am not understanding why am having different results

1. I created regular quantity calculation which is SUMX(Table1,Table1[quantity])

2. I created nested quantity calculation using quantity measure = SUMX(table1,[quantity sumx]) which is sumx & then my previously created measure quantitysumx

3. I created nested quantity calculation using SUMX(Table1,SUMX(Table1,Table1[quantity]))

so my question why items 2 & 3 giving different results although both have nested sumx, to my knowledge sumx should iterate row by row, so when i have nested , then for example if i take product a first sumx should be 10+3, then second sumx should be again 13+13 as it is row by row , but when am using measure instead of sumx , its giving different result

i attached below results 

[tamerj1 · Accepted solution]

Hi @chahineatallah 
The reason is context transition. By default all measures are wrapped with CALCULATE forcing context transition. In your case I can tell that products A and B each has two rows while product C has only one row. In the case on nested SUMX and two rows per product the result will be doubled as for each row the inner SUMX will produce the sum of the two rows visible in the current filter context. While when referring to a measure or when wrapping the inner SUMX with CALCULATE then each row or the outer SUMX will be transformed into a filter context that will bring the rows of iteration of the inner SUMX down to only one row. 
This is why when there is only one row per product the result of the two measures is the same.