The European Commission Recommendation specifies that a critical subcontractor or a crucial supplier must be audited “if this is likely to ensure more efficient control… in particular if the main part of the design development, manufacturing, testing or another crucial process is located with the subcontractor or supplier” (clause 2, point c and Annex III, point 2). The official definition of "critical supplier" is provided by the Notified Bodies Operations Group (NBOG) Guide ‘Guidance for Notified Bodies auditing suppliers to medical device manufacturers’ (NBOG 2010- 1).
2.2 Critical supplier
A critical supplier is a supplier delivering materials, components, or services that may influence the safety and performance of the device.
Note: In the context of the audit of medical device manufacturers, a critical supplier is a supplier of a product or service, the failure of which to meet specified requirements could cause unreasonable risk to the patient, clinician or others, or could cause a significant degradation in performance. This can include suppliers of services, which are needed for compliance with QMS or regulatory requirements, e.g. internal audit contractors or Authorised Representatives.
The usual interpretation is to consider that
A critical subcontractor ensures all or part of the MD's design, or performs all or part of the manufacturing processes, or carries out all or part of an activity in relation to regulatory requirements (e.g.: post-market data collection), and
A crucial supplier provides finished devices, or key subassemblies essential to the performance of the MD, or critical raw materials.
The manufacturer must provide the Notified Body with the list of critical subcontractors and crucial suppliers as per their risk management system. This list is reviewed during the planned audits of the certification cycle.  There is no regulatory requirement for the critical subcontractors and crucial suppliers to be informed of their inclusion on such a list.