Every measure on a shelf is aggregated, and Sum is only the default. Right-click a green pill and open Measure (Sum) to switch to Average, Median, Count, Count (Distinct), Minimum or Maximum. The pill text changes to match: AVG(Sales), CNTD(Customer Name).
Count counts rows. Count (Distinct) counts different values. A customer with eight order lines counts eight times with Count and once with Count (Distinct). You can count a dimension too: drag Customer Name to a shelf, right-click it and choose Measure > Count (Distinct).
Ratios need aggregate calculations. SUM([Profit]) / SUM([Sales]) divides the totals of each mark. [Profit] / [Sales] divides row by row and then adds the ratios up, which gives nonsense. Mix the two styles, as in IF [Region] = "West" THEN SUM([Sales]) END, and Tableau stops you with "Can't mix aggregate and non-aggregate arguments".