Create alternate price list (Alt. PL) items will create new Alt. PL items and over-write existing ones. Update alternate price list items will only modify existing Alt. PL items.

When using the Create function, you have the opportunity of simply copying the price/cost amounts from the master item or changing the amounts up or down by a percentage. You can also select which of these amounts are brought in unchanged and other amounts are modified.

When using the Update function, you can only change one amount at a time (say Base Price or Sale Price, but not both at once). Updating can be done based on either the master item or the Alt. PL item. You can update using an amount, percentage, markup over cost, or required margin.
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If you require very specific creation of prices and costs that don't follow a general pattern, import is a better way to go. If you simply want to copy over the master item amounts to create Alt. PL items (for special price recognition), then Create/Update is the way to go. Make sure that you don't overwrite amounts on the Alternative Price List that you want to keep.