Personally, I have been using Fiddler and Postman to upload bulk data (it seems that Fiddler allows larger JSON objects) and it worked perfectly. When uploading multiple records in your content type you must specify an array that contains all JSON records as outlined here
I have been uploading w/o hassle through a POST call ~ 1200 items (Fiddler), ~ 500 (Postman). The numbers w/ Postman are relatively smaller due to some restrictions for the JSON content length in this tool.
Regarding the second part of your question, a success result will return you the Id(s) of the created item(s) and this is
created in the backend. In the data browser of your "ICDCode" content type there is a grid and also a detail view (a pane in the right part of the window) that shows details when a record in the grid is selected. To see not only the predefined fields
in the grid define the structure of your content type by choosing it and pointing the 'Structure' tab in the right corner of the UI. Now add fields that match the fields "Code", "Description", etc., and the Backend Services will map the JSON data with this structure. Otherwise the data is still explorable but only when you select a single item - in the right pane you will see its details as an expanded JSON object (w/ very handy editor) and a message similar to "Fields that are not defined in the structure...".
I hope that this information will help you get started.