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So for example the GRIB edition 2 local parameters for DWD (centre=edzw) are defined in definitions/grib2/localConcepts/edzw and GRIB edition 1 local parameters for SMHI (centre=eswi) are in definitions/grib1/localConcepts/eswi.

The following in is an example of the contents for the above files in order to handle ECMWF parameter Geopotential in GRIB edition 1:

#Geopotential
'Geopotential' = {
    table2Version = 128 ;
    indicatorOfParameter = 129 ;
}

#Geopotential
'129' = {
    table2Version = 128 ;
    indicatorOfParameter = 129 ;
}

#Geopotential
'z' = {
    table2Version = 128 ;
    indicatorOfParameter = 129 ;
}

#Geopotential
'm**2 s**-2' = {
    table2Version = 128 ;
    indicatorOfParameter = 129 ;
}

And for GRIB edition 2, the contents of the same files for parameter Potential temperature gradient arethe ECMWF parameter 'Potential temperature gradient' in GRIB edition 2:

#Potential temperature gradient
'Potential temperature gradient' = {
    discipline = 192 ;
    parameterCategory = 129 ;
    parameterNumber = 3 ;
}

...

#Potential temperature gradient
'K' = {
    discipline = 192 ;
    parameterCategory = 129 ;
    parameterNumber = 3 ;
}

Install and test your changes

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% grib_ls -jn parameter out.grib2

You should get something like this:

{
  "centre": "ecmf",
  "paramId": 129003,
  "shortName": "ptgrd",
  "units": "K",
  "name": "Potential temperature gradient"
}

Also please study the slides here:  Training course presentation.
And the How-To Article GRIB: Converting edition 1 to 2 (The "Local configuration" section)

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