You can find this setting in Support → Custom Fields → Customer fields.
As the name suggests, customer fields are fields related to customers.
You can add custom fields for the customer fields.
Customer field is available to add in ticket form fields only when it is allowed.
Customer field is available in the My Profile section of the support portal only when it is allowed.
Customer can edit customer fields in the My Profile section of the support portal.
Agents with the capability Edit customer info can update customer fields in an individual ticket customer info.
Site administrators can edit customer fields in Support → Customers.