The key element to be taken into consideration is the contractual power of the sending organisation with regards to the seconded persons.
The existing link between the sending organisation and the seconded person should legally allow the sending organisation to give instructions and supervise the seconded person in order to ensure compliance with RISE obligations (ex: full-time work, confidentiality, IPRs, use of resources, etc…see especially articles 6 and 32 of the GA). This is normally the case with employment contracts but, as in IRSES, other forms of contractual link are accepted provided they ensure to the organisation the legal means mentioned above.
For example, in FP7 IRSES, doctoral students were treated as staff members, as they had a doctoral stipends/fellowships, which is also a kind of contractual obligation. But it is good to double check in case the doctoral student in questions does not have a stipend/fellowship or other contract.
(Grant Agreement and NCPs)