I don't know exactly what VSL had in mind for this filter but I regard it as a handy "distance increaser" - probably most relevant to those who reconfigure their orchestra's stage seating plan, or who want a somewhat more distant listening position.
If you want to move an instrument or section further back on stage, and/or if you want a listening position further back in the auditorium, then first of course apply volume adjustment (-6dB for each doubling of distance to the listener), then just bring the filter in very gently and adjust it by ear until the relocated source sounds right for the new increased distance.
Within the confines of a stage there's no more than a very subtle high frequency roll-off effect due to air absorption. But there may also be some 'screening' effect due to other musicians partly in front of the new playing position, or due to more audience members between the orchestra and the new listening position, to take into account.
The filter (left at its -6dB/octave default setting) may help very nicely in these scenarios if used sensitively.
If you're wondering about the degree of effect of the filter, I'd suggest first trying some experiments with a spectrum analyser (such as those built into many EQ plugins these days), so that you can see if and how the filter is having an effect on your selected sources.
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