ISO 9001 is the international standard that defines the requirements of a quality management system (QMS). In its 2015 version, it rests on the process approach, customer focus, management commitment, risk-based thinking and continual improvement. It says nothing about the technical characteristics of a product: it requires the organisation to control the way it works.
A “certified” company has been audited by an independent certification body, generally accredited (in France, by Cofrac, the national accreditation body). The certificate covers a precise scope, activities and sites, and follows a three-year cycle: initial audit, intermediate surveillance audits, then a renewal audit. In practice, a certified workshop has procedures for contract review, purchasing, calibration of measuring equipment, handling of non-conformities, corrective actions and batch identification, that is, traceability.
It should not be made to say more than it does. Certification is not a quality label for parts, guarantees neither a tolerance nor a material, and is not mandatory: many competent workshops are not certified, often because the cost and administrative burden weigh heavily on a small business. Conversely, an expired certificate, or one whose scope has no relation to your need, proves nothing. More demanding sector-specific versions exist: EN 9100 for aerospace, ISO 13485 for medical devices, IATF 16949 for automotive.
In a quote request
If your specification or your own customers require it, state that certification is required and ask for the certificate: body, scope, expiry date. Otherwise, do not impose it by reflex, as you would rule out good workshops; ask instead about their inspection, traceability and handling of deviations, for example around the first article. The guide on ISO 9001, EN 9100 and ISO 13485 certifications helps you decide, as does the one explaining how to choose an industrial subcontractor.