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The standard bass notes, with additional octave up strings. Unusual, but not rare. Hagstrom made one back in the 60’s that was used by both Noel Redding and Jimi Hendrix, then Tom Petersson of Cheap Trick took the concept further by adding yet another octave with a 12-string. Jeff Ament of Pearl Jam picked up on it as well.
The point is to fill space with guitar-like tones. I had an 8, made by Wolf, great instrument, but you really have to dedicate yourself to the technique changes, so I passed it along and just use octave boxes.
Currently, the most recent 8-string player I know of is Sara Johnston of this goofy Canadian band, the Boojums…