It is no longer a rating. It is a certificate.
From Annex I (Part-FCL) to Commission Regulation (EU) No 1178/2011 ("The Aircrew Regulation"):
FCL.940.FI FI - Revalidation and renewal
(a) [...]
(b) [...]
(c) Renewal. If the FI certificate has lapsed, the applicant shall, within a period of 12 months before renewal:
(1) attend an instructor refresher seminar;
(2) pass an assessment of competence in accordance with FCL.935.