If a staff style consists of more than two staffs, the brackets and bar lines can be set to connect only certain parts of the overall staff style—they can be interrupted in between staffs. If you want to delete any connections, grab the symbol at its end (bottom), and drag it up, until it disappears.
The voices value in the style overview is determined by the number of voices in the staff style. This can be changed in the Voice parameters section, but not in the style overview.
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Logic Pro User Guide: PDF