With offline conversion measurement on Facebook, you can track when transactions occur in your physical retail store and other offline channels (ex: orders made over the phone) after people see or engage with your Facebook ad.

Link for more info:
https://www.facebook.com/business/help/1782327938668950

Set up an Offline Conversion:
1.  Login to your Facebook Business Manager, go to your Business Manager menu then click OFFLINE EVENTS

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2. Click on Create a NEW OFFLINE EVENT

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3. Give your Offline Event a Name

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4.  Select the Accounts you would like to create the Offline Event

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5.  (Optional) Give users permission to upload the OFFLINE EVENTS csv file

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6.  After you have create the CSV file, its time to upload. Be sure the CSV file follows the correct format  (download)

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7. Upload file. Click here for more details

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