How DAX Difference between naked SUM and sum wrapped in CALCULATE

Question

What is the difference between naked SUM and sum wrapped in CALCULATE?

Measure1 = SUM( tab[col1]) )
Measure2 = CALCULATE ( SUM( tab[col1]) ) )

Answer 1

This is materially different than the question you asked, because you were asking about two fully defined measures, not two snippets of DAX. The measures will behave identically, because their expansions are logically equivalent:

//Measure 1 definition:
Measure1 = SUM ( 'tab'[col1] )

// Measure 1 expands to the following, and you would use this if you were
// replacing a reference with code:
//Expansion1:
CALCULATE ( SUM ( 'tab'[col1] ) )

//Measure2 definition and expansion:
Measure2 = CALCULATE ( SUM ( 'tab'[col1] ) )

//Expansion2:
CALCULATE ( CALCULATE ( SUM ( 'tab'[col1] ) ) )