In order to retrieve all the tags within a specific classification you will have to use the listget web service, which is included in the itagdefinition package of xintelligence.
The url is structured as follows:
Where clientId is the domain name used to access your THRON (usually your company name), while classificationId is the ID of the classification for which you want to extract the tags; this ID can be retrieved with a listget web service which is included in the classification package of xintelligence. Remember to include a valid token among the headers of the request (user performing the request must have the permission to view the tags within that specific classification). Within the response, for each tag you will be able to read both its "id" and its "label".