what they Meant by:
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Yamaha is also granting software developers permission to use voices from its electronic musical instruments for creating their own DLS data, giving developers access to many special voices not included in the XG voice library.
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it mean that someone can resample whole YAMAHA product to DLS format?

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what they meant with offer open acces to XG and GS format?

is this about resampling sounds from hardware instruments output?

it means exactly what it says. Yamaha are granting SOME software developers permission to use some of their sounds. If you have not been granted permission or been offered to use some of their sounds then this obviously does not apply to you or anyone else that has not been specifivcally granted permission.