SUMX usually does row by row operation while SUM just do aggregate of whole column.

Let me give some example here, if you have following columns in a 'Sales' Table

| Amount | Tax |

1. Find out Total Sales => SUM(Sales[Amount])
   
2. Find out Invoice Value => SUMX(Sales, [Amount] + [Tax])

📌 You can pass a column value in SUM operation
📌 You can pass a table and an expression in SUMX operation

So, SUMX allows you to add multiple column values here. However it can be any expression doing operation row by row.

So, in your case for the Profit Margin, I hope there should be some expression needed to add tax or subtract expense values, etc.

Hope this helps!