[gdal-dev] reference for tags used in metadata inside GeoTIFF

[Eloi]

Hi all,

Is there a reference for tags used in metadata inside GeoTIFF files?
I would like to add a legend to the metadata, but before proceed I would 
like to know if there is a standard way of doing this.
By now I came up with two options:

     gdal_edit -mo "LEGEND_VALUE_1=A" -mo "LEGEND_VALUE_2=B" (...)
Or
     gdal_edit -mo "LEGEND=1:A, 2:B, (...)"

Is there a 'best practice' to put a legend in a GeoTIFF? Thanks.

Cheers,

Eloi