Eccodes grib_copy - How to identify the levels and parameters?

I would like to generate a grib from a full GFS grib2 having, for example, only the U and V component for 850 hPa. To do so, I’ve seen I could use the eccodes library or the command line grib_copy but I don’t find how to filter the parameters. Usually in GDAL I use the band numbers but with eccodes I’ve no idea where to start.

Do you have any reference that tells me how to get the parameters? Or maybe another way to do it?

In the grib_copy documentation I couldn’t find (https://confluence.ecmwf.int/display/ECC/grib_copy)