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The purchase order number set for expenses on the placement , isn't show shown on the expense claim form but is automatically stored against it.
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Purchase order numbers can be directly applied to expense items which will override any purchase order number set on the placement.
Purchase Order Number on Invoices
You can set up invoices to show the purchase order against individual items depending on your consolidation and layout settings or at invoice level. When invoicing items which have different purchase order numbers these will appear as 'various' at invoice level but can be displayed by line depending on your agency settings as shown below.
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