If a beneficiary wishes to hire a researcher who has resided in a certain country (see here for more information about these countries and sanctions of the EC) it should be noted that the beneficiaries are only under a legal obligation to comply with EU restrictive measures ('sanctions') as such. From case to case – and not systematically – the Research Executive Agency will ask the coordinators of specific projects to provide such a 'Clearance Certificate', as this is in many cases the easiest and fastest way to certify compliance with these restrictive measures (as long as the certificate is easily obtainable enough in the respective country).
With regards to the nature of the clearance certificate, this is a general term used for a document that a researcher wishing to work on restricted activities in another country must obtain from the competent national authority before they take up work. In the case of, e.g., the MSC-ITN actions, the clearance certificate is a certificate that the recruiting beneficiary needs to request from the competent national authority that will state that by letting the researcher working on the project, this will not be in breach of EU restrictive measures that are in place.
Should the acquisition of a clearance certificate however lead to difficulties, then it would be best to advise the beneficiary/coordinator to communicate this to the responsible Project Officer at the REA so that they are aware of the difficulties and can - if deemed necessary for the specific project in question - explore other ways to ensure compliance with EU restrictive measures.