From the Government of Mexico - Draft procedures for conformity assessment of Norma Oficial Mexicana NOM-028-STPS-2004, Organization of work-safety processes chemicals. A google translate version of the amendments here:

That pursuant to the provisions of Articles 73 of the Federal Law on Standardization and Metrology and 80 of its Rules of Procedure, it is necessary to establish procedures for conformity assessment of standards Mexican official, when required for official purposes of verifying compliance the same;

That conformity assessment is determining the degree of compliance with the Mexican official standards and includes among others, the sampling procedures, testing, calibration, verification and certification;

Click on the link above for the Google translate version, or click for the Spanish version.