Hello Lalit
I agree with Mukesh's response.
As all the Univariate models are basically Time Series models. Time series models use patterns from the past to try and predict the future(extrapolation). These models required number of periods for forecast model initialization and to identify the data pattern, and so it has an impact on history horizon.
Following is how I have understood model initialization period required by forecast models -
Constant Model requires ...1 Period...because it has to just use that one period and then extrapolate into future.
Trend Model requires - 3 Periods...to know whether trend is increasing(upward) or decreasing(downward) minimum 3 periods are essential.
Seasonal - 1 complete season...as this will only tell the system how the system shape up, which all months it rises or contracts and which all months it flattens..
So obliviously for Trend + Seasonal...1 compete Season and 3 Periods becomes essential.
Here is the SAP Help link on this topic
Model Initialization - Forecasting - SAP Library
Hope this will help.
Thank you
Satish Waghmare