[A] In order to fetch the custom field data in next document, you have create the custom field in next document.
[B] We will take an example, here we will see how to fetch the custom field of Sales Quotation(SQ) to Order Confirmation(OC).
[C] In order to add custom field in OC go to "Settings module >> Custom field" and select the order confirmation. now click on "Add Field" option.
[D] Now select the "Data type" same as the previous document custom field
[E] Select the desired custom field from previous document and click on the "Save Field"
[F] Now your custom field is saved that will automatically fetched from SQ to OC.