FILTER() is most often used as a filter parameter within the calculate function. OK, so now it is getting confusing. FILTER() is a function. There is another use of the word filter in DAX, and that is as a parameter in CALCULATE. Let’s look at the syntax
FILTER Syntax: FILTER(<table>, <true/false test>) CALCULATE Syntax: CALCULATE (<measure expression>, <filter parameter>, <filter parameter>,…)
This is one of the most useful functions in DAX and one you really want to know well.