The 'order.source' refers to the integration that created a Sales Order. This trigger/id is always provided on an Order and applies to the platform from where it was imported.
For platforms on which a customer may have more than one store, i.e., an account with 2 Shopify stores, the'order/channel' refers to the particular store where the order was created.
The 'order/channel' trigger/id is newer and is only found on internally-built, and some of the newer, and externally build integrations.