OIML R 136-1 - Organisation Internationale de Métrologie Légale
https://www.oiml.org/en/files/pdf_r/r136-1-e04.pdf
Maximum permissible errors (MPE). 2.3 .... metrological controls applied by the national metrological ... Recommendation, the definitions below apply. T.1.

R 117-1 - Organisation Internationale de Métrologie Légale
https://www.oiml.org/en/files/pdf_r/r117-1-e07.pdf
other than water. Part 1: Metrological and technical requirements .... Maximum permissible errors and significant faults on calculators ....................................27. 2.9. Indications . ..... EM = electromagnetic ... MPE = maximum permissible error.

See terminology on page 2 of link above
MPE = maximum permissible error
EM = electromagnetic

Makes sense?
HTH


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Note added at 39 mins (2016-06-17 15:48:04 GMT)
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Marta:

Can we have more context please? I am not sure about "duplex accuracy", nor "EM" without more information. Thanks.

Is "duplex accuracy" defined elsewhere in your document? I've found other references to "dual accuracy", so I don't understand "duplex".

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Note added at 50 mins (2016-06-17 15:59:02 GMT)
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http://www.pmargage.com/kobastepmore.shtml
KOBA-step® Gage Bar (KOBA-step is a registered TM). "Gage", although I would have said "gauge" (British English), so it is assumed your text is American English?

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Note added at 55 mins (2016-06-17 16:04:02 GMT)
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http://publications.npl.co.uk/npl_web/pdf/mgpg42.pdf
Kolb & Baumann GmbH & Co. KG
(website: www.koba.de/ index.php?lang=english) are manufacturers of gauge
blocks, ball plates (KOBA-check), ball bars and step gauges (KOBA-step). They can
be contacted at:
Kolb & Baumann GmbH & Co. KG
Fabrik für Präzisions-Messzeuge
Daimlerstraße 24
DE-63741 Aschaffenburg
The UK distributor is OPUS

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Note added at 58 mins (2016-06-17 16:07:06 GMT)
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http://www.koba.de/index.php?id=29&L=1
KOBA Ball Bar (although this is not used in your case)
one-dimensional, removeable test piece with spheroidally targets

scope of application:
- monitoring and traceability of large volume CMM's, e.g. horizontal arm and bridge type CMM
- checking of opto-tactile measurement systems like Lasertracker