N trend measures the difference between a period N and N-1, N-1 being in this case the previous adjacent period.
Depending on the analyzes, the comparison period will be
Weekly: the previous week (s37 vs. s36)
Monthly: the previous month (September 2017 vs. August 2017)
Quarterly: the previous quarter (July 2017 to September 2017) vs. (April 2017 to June 2017)
Annual: previous year (2017 vs. 2016)
In evolution modules, like Sales Evolution, the N Trend is calculated in Year N vs Year N-1, while that the N Evol percentage of the current year is the result of YTD N vs YTD N-1, (see screenshot below)
In the same way, the N Trend of a current Qtr (therefore of a QTD) in Sales Evolution, will be calculated vs the whole of the previous QTR.