Custom fields are also available for orders and their associated information. An order object in Shopify contains references to the ordering customer as well as the ordered products and variants.
Examples of Liquid drops to use in e.g. a Shopify notification template:
Please see this Shopify article on which variables can be referenced from within an order notification:
Notification variables reference
Shopify Help Center
This applies to Shopify's internal notification template. If you want to use the Order Printer app, metafields will not be available, so another approach is needed - please see this article from Shopify