First, resolution and accuracy are not the same.
An 6Digit meter gives no indication of its accuracy , for all you know a 5digit meter may be more accurate then the 6digit meter.
accuracy has a connection to a calibration standard. How good can this meter report true 1Volt dc for example ?
To maintain accuracy instruments must be calibrated over time.
Measuring Instrument state their accuracy within a calibration window ( 1 year will be less accurate the 3month calibration window)
Which time window you will calibrate depends on what accuracy you must meet for your products test specification.
When you get a project and you are given spec to meet(+/- limits) you measuring device must be X time better the the spec.
Look over all the spec which will be using the same measuring instrument(ex via switching matrix) so the accuracy is sufficient for all involved.
Calibration departments keep records to know when an instrument time has come up for calibration.
The Calibration department typically have even higher accuracy equipment or send equipment out to manufacturers or calibration facility.
A national standard is the ultimate source of reference and the paper trail will show a connection from a national standard to your measuring equipment.
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