Custom fields are arbitrary key value pairs that you can use to upload your own categorization for the uploaded custom data. For example, if you upload product reviews data, you can use a custom field to upload the product rating for each uploaded review.
When using custom fields, please note the following: only JSON string values are supported; custom field names' length needs to be equal or lower than 100 characters; custom field values' length needs to be equal or lower than 10,000 characters. You can use up to 10 custom fields per custom content source. If you need more custom fields, please contact our support team.
The JSON example for the Upload Custom Documents endpoint includes a document with 10 custom fields.