Why does MAXX not add the row context to filter context without CALCULATE?

Question

MAXX(CUSTOMER, SUMX(SALES,SALES[AMT]+0))

The above doesn't add customer into the folder context when performing the SUMX

Where as the following does:

MAXX(CUSTOMER, CALCULATE(SUMX(SALES,SALES[AMT]+0)))

Why does MAXX not add the row context to filter context without CALCULATE?

Answer 1

Calculate creates a context transition in this case.

https://www.sqlbi.com/articles/understanding-context-transition/