You have the ability to create Invoices from Quotes and PO's from Invoices. Now you are also able to tell which documents are linked to each other. (Available in the Full-Service Plan)
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For instance, if you created an Invoice from a Quote, you are now able to tell specifically which Invoice and Quote are linked without jumping back and forth from one tab to the other.

How do I know which documents are linked?

When you open any one of these documents (Quotes, POs, Invoices), you will now notice an option to view all the connected documents along with a standard link count to let you know the exact number of documents that are linked to the one you are editing.

To view the linked documents, simply hover over the linked option to see a quick preview

You then have the option to click on any of the document links to be taken directly to that document if necessary. Suffice to say, you can use that same method to work your way back.