https://help.myob.com/wiki/x/US_EAQ
Extra fields record additional information about a contact, client or employee where the value can be different in each instance.
Extra fields are displayed in the Extra tab on the Contact, Client, Supplier (AE), Assignment Details (AE) and Employee pages.
In AO Classic, extras fields were known as UDF's
User defined
Extra lists
Extra types
Creating, editing and deleting Extra Fields
Adding/entering extra field information on a client