Let's say I have a table like this - [Orders]
Date Amount Category
12/12/2017 100 A
12/12/2017 200 B
12/12/2017 300 C
1/1/2018 400 A
1/1/2018 500 B
And I have a slicer for Category.
If only one single Value is selected, then a measure like this will work
CALCULATE(SUM(Orders[Amount]),FILTER(ALL(Orders), Orders[Category] = SelectedValue(Category))).
When more than one value is selected, how would you pass that inside the DAX Measure?