Ongoing Warehouse pulls article registry updates from Magento to the WMS. Magento contains the master data.
The WMS downloads new and updated orders which are not yet handled. If an order is updated, the change is transferred unless the warehouse workers have started working on it. When orders are fulfilled, the WMS creates a shipment in Magento with the delivered quantities and the tracking information.
Stock balance is sent from Ongoing Warehouse to Magento. The WMS should be master data when it comes to available stock. This is especially important if you sell across multiple channels.