PowerBi DAX equivalent for SUMIFS with current row value as filter

Question

I am a table 

Month, CustomerId, ProductId, TotalQuantity
Jan,1, CAR,
Feb,1, CAR,

So I want to add sales into it

I add wd the column with the following formula

=SUMIFS(Sales[Quantity],Sales[CustomerId],[@[CustomerId]])

What is the use of @ from the above formula? 

Answer 1

Use the following Measures

TotalQuantity = CALCULATE(SUM(Sales[Quantity]), FILTER(Sales, Sales[CustomerId] = EARLIER (Sales[CustomerId])))

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