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There are two groups of people who can receive these emails. One group is your finance reviewers and the other is your PM reviewers. To learn about invoice approvers in general see the permissions section of the Invoicing help page.
Because the list of recipients has the potential to be quite large, we limit the email to the first ten email addresses identified.
Finance Reviewers
A user belongs to the finance team if they have the cost center permission Create and Approve Invoices for all engagements on the new invoice.
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