Custom fields allow for customizing the onboarding flows or adding these custom fields to documents. 

Guidance on creating a custom Flow: Here

Guidance on creating a custom Document: Here

To Create a Custom Field: 

1. Select Onboarding

2. Select Custom Fields

3. Select Add new Field

4. Create a name/label for the field

5. Select the type of field from the list of options

6. Add a description of the field

7. Save custom field