Open the invoice you want to review, then select its "History" tab:
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Invoice status changes: created, sent for approval, rejected, approved, paid, and other status changes:
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Rate changes: rate changes applied to invoice entries, with details:
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Notes: notes added to entries on the invoice:

Detached entities: an entry detached from the invoice, once the invoice is no longer in Draft status:

If an expected change isn't in the history, contact Boostlingo Support with the invoice number and the change you expected to see. If you're unsure how to reach Support, contact your Account Manager.