When used in other supported stages, $sum has one of two syntaxes:

• $sum has one specified expression as its operand:

{ $sum: <expression> } 

 

• $sum has a list of specified expressions as its operand:

  { $sum: [ <expression1>, <expression2> ... ]  } 

   

Only the 2nd syntax is implemented, because we thought it functionally covered the first with a single expression array.

But in the 1st syntax, the expression can resolve to an array.

Example : https://www.mongodb.com/docs/manual/reference/operator/aggregation/addFields/#using-two--addfields-stages