To achieve certification, the standard requires support of the ads.txt initiative, adherence to the LEAN standards set by the Coalition for Better Advertising and JICWEBs DTSG certification for brand safety.
Most of them are accredited, or at least audited, by independent organizations on the markets they want to operate such as the Media Rating Council (MRC) in the US, the JICWEBS in the UK, the CESP in France.