Hello Shyam,
Forecast Horizon is number of days when the forecast is not considered as part of the total demand. Beyond this horizon, SNP calculates total demand as either the forecast or the sales orders (whichever is greater) plus the other demand (dependent and distributed demand, planned and firmed). Note- Forecast Horizon is a key in the calculation of Total Demand.
Most certainly Forecast horizon is considered during the heuristic run and in interactive planning too.
Hope this will help.
Thank you
Satish Waghmare