ISO/IEC 17065 "Conformity assessment — Requirements for bodies certifying products, processes and services" is the strictest available standard for product certification programs in general. The scope and definitions of this standard are freely available here.
The standard lays down rules how a third party (the certification body) can be accredited (be allowed) to certify products by a standard that is owned by the standard owner that itself is not allowed to issue any certificates.
Only programmes that are designed according to strict European rules formulated by the association of European accreditation bodies "European Accreditation (EA)" are suitable for this. The central document is EA-1/22, plus the rules of the German accreditation body DAkkS which is in the case of Level the so called "home accreditation body".
The procedures for establishing a product certification program are defined in detail. For example, a public stakeholder process is mandatory when setting up a new certification scheme and everything must be well documented and be thoroughly checked by the national public accreditation institution.
The official accreditation to ISO 17065 makes the Level certification schemes different from other not ISO-accredited schemes without formal public accreditation acknowledgement by national bodies as DAkkS in Germany, ANAB in USA, Accredia in Italy or ENAC in Spain. Find all the responsible European accreditation bodies here.