When I fetch an order, I need to get the ItemLookupCode for each item, which is the unique identifier that we use across the platform - there is an 'ItemID' which I discovered from using the API but that's "internal"

So, to get the ILC means a further (number of order items) API calls to /Item/Id based on the 'ItemID' property referenced in the OrderLines array

It'd be better to do this at the server side in a database join instead of several HTTP API calls.