ITN: Is subcontracting possible in ITNs and if yes - covered by which cost category?

Yes, subcontracting is allowed provided that only minor scientific/training tasks are concerned and beneficiaries award a service contract for which they can either pay out of the Research, Training and Networking Costs-category or the Management, Indirect Cost-category (depending on the subcontracted activity). Remember that beneficiaries have to show that they have the capacity to carry out the project activities (see latest ITN-Guide for Applicants). Coordinator's core tasks can not be subcontracted, like for example subcontracting management tasks (see ITN model grant agreement, art.41.2(b)).