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_extId
When you open a new form via, for example, the Link (Email/Web Page) share URL, you will always open a new form instance. As you use the form and enter values into the form, the values are immediately cached on the Form Server as you tab to the next form field. If you close the browser tab/window containing the form prior to submitting the form, the Form Server still retains this form instance. By using the special (reserverd) URL parameter called _extId, it is possible to reconnect to an in-memory form instance.
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