Many Universities recruit early stage researchers for the PhD position and are salaried according to ITN guidelines. Moreover, they are employed as staff. But, to get a PhD degree, i beleive that the candidate may also be registered as a student with the university. In such a case, are the students supposed to pay for the annual PhD tuition fee ? or they are exempted from tuition fee due to some ITN guidlines ?

In the UK, the tuition fee for the EU citizen for a PhD  is approximately £4,327 per year and for a non EU citizen, it is nearly £16,700 per year. If the tuition fee is not exempted for an ITN researcher or non EU citizens are asked to pay EU PhD rate, the non EU citizen's real earnings( assuming that the candidate pays the tution fee from his PhD position earnings) are much less than that of a EU citizen.  Ofcourse, it would be still more than the standard RCUK PhD Stipend rate by 45-50%.