Beyond the standard fields we understand (first name, last name, email, zip/postal code, city, state, address, country, and language), you can upload any other columns you want in your spreadsheet, such as phone number.
When you match your columns to our fields, you can choose to match these extra columns to your existing custom form fields in our database, or you can create new fields on the fly by clicking the "Create New Field" button and entering a title for your custom field, if you have the permission to edit custom fields.
Any custom columns uploaded this way will be available as custom form fields, just like named custom HTML form inputs. You can use these custom form fields for email targeting or for reporting.
The sample file shows an example of custom columns.