Weighing certificates confuse buyers because three very different documents get mentioned together: OIML approvals, ISO 9001, and calibration certificates.
OIML is the international body that publishes accuracy recommendations for weighing instruments. A scale built to an OIML recommendation (like R76 for non-automatic scales) has a design proven to hold its accuracy class — that's what makes it suitable for trade and billing.
ISO 9001 certifies the manufacturer's quality system, not the individual scale. It tells you the factory that built your scale runs documented, audited processes.
A calibration certificate belongs to your specific scale: it records the readings your unit produced against reference weights on a given date. Billing counters, labs and QC departments should recalibrate on a schedule — annually at minimum, more often under heavy use. AMC plans bundle this with preventive maintenance.
Hazardous areas add a fourth document: ATEX certification, which rates equipment as safe for specific explosive-atmosphere zones. If you weigh in a refinery, depot or solvent store, the ATEX zone class is the first question to answer.