A custom field is a key:value pair that is defined for a given product, which you can override for any license you issue or edit.
For more information on defining product custom fields, please read Product Custom Fields.
If defined, the key and the value of a custom field is always returned on License Checks and License Activations. You may modify the value for any custom field for each license.
License policies are templates you can use when issuing a license, and you can specify a license custom field for any license policy. Any license issued with the policy will use the license custom field
Open the license policy editor by navigating to the license detail page > license policies, and either "add a new license policy" of by clicking on the pencil of an existing license policy.
In the license policy editor, click on "custom fields" and "select the custom fields" from the dropdown. Tick the box of any product custom field you wish to override the values for. Their values (not their names) for those selected fields can then be edited in the table. Click confirm when you are done.
Open the license detail page, and click on custom fields. You will see a table with all of the custom fields for that product, regardless whether or not they have been overridden. Click the pencil on the row for the value of the license custom field you would like to override. Click "confirm" to save the changes: