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With this setting enabled all client data, including address, company name, VAT number etc will be stored during invoice generation and related with invoice. Browsing invoice from clientarea or downloading PDF will show that initially saved data
Updating client data on already generated invoice
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It sometimes would be required to update client details on invoice that already was generated with above setting enabled. To update this info go to invoice details in adminarea, click on Edit details and click on Click here to set client details on invoice with current client info as shown on image below
See a video guide here