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For .sif files, you don't need to set the vendor as it's determined by the MFG Code on a line-by-line basis.
The Discount Template can be used for applying Manufacturer set discounts based on contracts.These discount templates are setup in a separate window, please see SPA's and Special Discounting / Vendor Commissions. Choosing a template here will apply preset discounting to certain product types from certain vendors and / or apply a Vendor Commission Percentage to each line item that matches the template details.
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