Adding hidden fields to a list

Hidden fields are those fields that you can use to store information and are used later for segmentation. They are called "control fields".

In Contacts - Custom fields you have the option to create fields that you can configure as visible or hidden. 

Note that fields that are configured as visible will appear on the registration and update form that you offer to users.

You can also edit the visibility of the fields offered by the platform, not only the ones you have created: edit them from the Options panel with the icon and modify the visibility.

Once this process is done, you have to link the new custom fields to the list you want.