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To help understand this, let’s look first at the correct answer with the Ranking Context included:
Now let’s comment out the ranking context line:
Top 3 Salespeople per Region =
RankingContext = VALUES( Salespeople[Salesperson Name] )
CALCULATE( [Total Sales],
ALL( Salespeople[Salesperson Name] ),
[Total Sales] )
What this measure now says is “calculate the total sales for the top three salespeople, based on total sales and remove the filter (via ALL) on salesperson name, so that it does this calculation across all salespeople within a region”. By removing the ranking context, you leave the ALL statement in place and active, and so the value that will be returned is the same $27,676 calculated above, but with the filter on salesperson name removed, this value will be returned for every row within a region, and that’s exactly what happens:
Note that the total still changes by region, since there’s nothing that removes that filter context.
I hope this helps clarify things. I would definitely recommend going through the sections of this course that address evaluation, filter, and row context. Those are the essential concepts at play here.