When adding custom fields to ConvertFlow form elements, you can choose to select from "Infusionsoft fields". This allows you to map contact info entered into the ConvertFlow custom field, into the correct contact custom field in Infusionsoft.

When you create a survey element in ConvertFlow, click on the "Map to custom field" dropdown to the right in the builder panel, and you can select from your Infusionsoft custom fields.

On your website's Integrations page, when you have Infusionsoft connected, you'll see a "Fields" button, that when clicked, allows you to map ConvertFlow's preset contact fields to Infusionsoft custom fields. 

This is useful for mapping contact info added to ConvertFlow's preset fields through ConvertFlow's forms, that isn't already mapped to fixed fields in Infusionsoft. For example, "first name" and "email address" are already mapped to Infusionsoft for you. 

Just enter the data name of the Infusionsoft custom field you would like ConvertFlow's preset fields to be mapped to, and you're set!