In Modelling the strategies calculate from the average rate of demand of the selected date range. If you have very quiet off seasons and very busy peak seasons, then you may find the modelling calculation is unsuitable for extremes. Modelling strategies are ideal to set stock limits for fairly consistent levels of demand. 
Forecasting looks at each day of the future and checks if it has enough stock on hand that day to cover the min days of stock you set. If it forecasts that it does not have enough stock to cover the min-days of stock of the strategy, then it replenishes equal to cover the amount of demand it sees over the follow number of days equal to the max-days of the strategy. 

If you opt to use a manual strategy and enter the min/max quantity directly, then the forecast will operate in a similar and more limited way as to how Modelling works. As stock hits the min qty limit, it will replenish up to the max qty limit.