The 'Professor Emeritus' & 'Visiting fellow' policies can vary a lot from one host institution to another. The institutional link of this staff is normally quite clear (e.g. they are usually listed in the faculty/staff directory, they have full access to institution facilities, etc.). However, sometimes they require prior authorisation from the institution in order to apply for grants or participate in certain projects.
Hence the involvement of this kind of supervisors with the institution should certainly be clarified before submitting the proposal, in order to avoid any subsequent problems at the time of the GA signature in case of a successful proposal. However, please note that supporting documents are not requested by the REA at the time of the proposal submission. Applicants (researchers & institutions) are solely responsible for checking the fulfilment of all eligibility criteria and provide the correct information in the submission forms.