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Segment
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A segment is a group of item+locations, all satisfying a specific filter criterion.

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Field            Type              Description
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name             non-empty string  A unique name for this segment
description      string            A free text description.
query            string            | A SQL-like expression to identify the item-locations belonging to
                                     the segment. All item attributes, location attributes and buffer
                                     attributes are available to define the filter expression.
                                   | For example : item.cost between 10 and 100 and location.name
                                     like '%shop%' and buffer.onhand > 10
sku_count        integer           | The number of item-location combinations in the segment.
                                   | The value is read-only and is automatically updated when a
                                     segment is created/updated or when an inventory planning task is
                                     run.
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