What status codes, apart from the 200 OK, can be expected when making DSS REST calls for extraction of data?
I recommend you the reading of the Programming without SDK Tutorial. You should find the answer you need in the Managing the HTTP responses section (it contains a link to the exhaustive list of codes). In a nutshell, 200 means the data extraction is complete, 202 means it is pending.
Please, read this great tutorial for more details. It is available in the DataScope Select - REST API page where you'll find tons of other excellent learning materials.
The codes can be found on page: https://hosted.datascopeapi.reuters.com/RestApi.Help/Home/Index, or to navigate to this page, by going through your DSS GUI: 'Help' > 'REST API Help' > 'SDK' > 'Status Codes' (DSS credentials required).