What does different invoice status mean?

In Finances -> Guest Invoices, you have access to the list of all your invoices. You should click on an invoice to see its details.
In the invoice page, you may see different status for an invoice: Draft, Completed, Posted and Voided.
If the Doc Status shows Draft – the invoice is subject to new changes- you can click on Re-generate button to generate the latest version of the invoice.
If the Doc Status shows Completed – the guest checked out and the invoice changes were finalized– you can click on Post to Xero button. Please note that Post to Xero button is available only if you have activated your integration with Xero. For Completed  status, you can also click on Change to Draft button and click on Re-generate button to apply the latest changes.
If the Doc Status shows Posted – the invoice has already been posted to Xero- the Void button will appear to let you void the posted invoice.
If the Doc Status shows Voided – the posted invoice has been voided- you can apply the new changes, regenerate the invoice and post it again.