I suspect the offsets are referring to the samples and are a number added or subtracted to the sample number. Perhaps useful if your sample has a silent gap at the beginning. An offset could make it start to play when the sound really begins and not play the silent bit.
I agree with Bottrop, forget about Viena (it has only one 'n'). It is very old and Polyphone works much better. However I do not agree with him about stereo. All musical instruments produce stereo sound and if you do not record stereo samples your sound font will be inferior to one which uses proper stereo samples.
csw900