Apparently, the author wanted to parametrically set the point of division of the entire calculated range (Forecast:Month) into two parts for two calculation algorithms.
If anyone needs to emulate in QM this behavior, try applying this formula (see below):
On the left side of the formula you cannot have it reference another Matrix. A formula is matrix specific. With your example what you are in effect saying is In Forecast, Oct to Matrix Month Ending Month Mar which does not make sense. Hence, a syntax error.
What I think you may be getting at is that you want Forecast to be from Oct to which ever Month is listed in Month::Month Ending, but I’m not sure.
So can you tell us what it is you are trying to do or want to accomplish?