It is originally a US standard intended to assess the suitability of materials for oilfield equipment where sulphide (sulphide) stress corrosion cracking may be a risk in hydrogen sulphide (sour) environments. However, the world standards body ISO has issued it under its own "brand". The latest edition includes technical corrigenda from 2005. Discussions about the standard can be found on the NACE website.
The standard specifies the types of corrosion resistant materials including stainless steels that can be used in specific oilfield environments and places limits on the hardness of the material. This applies both to parent and weld material. The maximum hardness is usually defined in terms of the Rockwell 'C' scale.
No conversion to other hardness scales is given in MR 0175 which presents one problem as softened stainless steels hardnesses are measured using either the Rockwell 'B', Vickers or Brinell scales.
Approximate conversions are available.
Details of the requirements of NACE MR0175 / ISO 15156 can be found on the following page.