Hybrid participation is common. A supplier may receive purchase orders and send invoices by EDI, but struggle with documents that need more detail, such as an advance ship notice with carton and pallet contents, compliant shipping labels, order changes or claims. Those steps then fall back to email, spreadsheets or phone calls.
The gaps matter because they tend to be the hardest documents. A missing or inaccurate ship notice can delay receiving and trigger chargebacks. A change agreed by email may never reach the ERP. Treating a hybrid trading partner as fully EDI-enabled hides these problems; treating it as non-EDI throws away the integration that already works.
The practical approach is to keep EDI where it works and give the trading partner a governed way to handle the rest, under the same rules and with the same references, so every document still ties back to the original order.