Regulation 64 establishes the rules surrounding the use of official lists and certification by certification bodies that contracting authorities may accept in a public procurement procedure. Albert's long entry is here.
This is an interesting Regulation as most of it pertains to England, Wales and Northern Ireland but concludes in paragraph 10 stating that there are no official lists of approved economic operators or certification by certification bodies in these jurisdictions. In other words, it appears it only exists to enable economic operators based in other Member States to use lists and certificates from said Member State.
By and large the logic of this Regulation is to reduce the transaction costs to both parties. Why on earth the lawmaker decided to not take up this opportunity for simplification and reducing the transaction costs in procurement is beyond me. Maybe a case of "not invited here syndrome"? After all the Directive does not include a mandatory obligation of creating approved lists or certifying bodies.
The savings in transaction costs occur by enabling economic operators to undertake the work of being included on an approved list or certified by a third party body. The contracting authority also benefits from the reduction as it will only have to check for the validity of the certificate or inclusion in the list, effectively outsourcing the cost of checking the underlying evidence to a third party, at least in first instance. The contracting authority may ask for the underlying evidence if there is justification to do so (paragraph 3).
As with standards in technical specifications, the participation and certification of economic operators is subject to "or the equivalent test", ie they cannot be forced to be in said list or have the certificate but can prove they comply with the requirements covered by them (paragraphs 7 and 8).