What is it?
If you wish to cumulate a measure relative to a dimension (the most common example is using time for year to date) then CUMULATE. CUMULATE may also include a start (count of items back to cumulate) and an end (count of items forward to cumulate) variable. You can CUMULATE across multiple dimensions simultaneously and/or combine CUMULATE with other functions.
How to write it?
Fx layout: = CUMULATE(Table.Measure,Dimension1,Start count, End count)
Example = CUMULATE(Sales Volume,Time.Month,Cumulate back in months to start,Cumulate forward in months to end)
Will result in the sales volume being cumulated over months, across the range based on the start and end range.
Knowledge Base link: https://www.arithmix.com/knowledge/measures-everyone-should-know