For Seasonal Analysis Interval, the computation will be based from last year and the months moving forward. For example, the computation was made on 10/11/2019, and the Seasonal Analysis Interval was set to 3 months, it would start computation from 10/11/2019 and consider the months of December, January, and February. This preference would be applicable for those items with the Seasonal Demand checkbox marked on their item record.
As for the calculation for Lead Time, given that it’s set to Auto-Calculate, this would be calculated by taking the number of days between the order entry and receipt on the three most recent purchase orders, and dividing by three. If more than three purchase orders exist, all purchase orders within the period specified in the Order Analysis Interval field on the Inventory Management Preferences dialog will be used. The divisor which is three should not be assumed as the minimum Purchase Orders for the system to auto-calculate the Lead Time.
If there is only one fully received Purchase Order within the Order Analysis Interval, the system will still auto-calculate the Lead Time.
If the Auto-Calculate box is cleared i.e. manual calculation, and no value is entered, then the default value from the Set Up Inventory Management page is used.