That's the edit cursor. For example, if Studio One is playing and you click somewhere with the smart tool on, you will see the edit cursor mark that location. If you fire (for example) split, it will split at the edit cursor, not the play cursor. When you stop playback the edit cursor disappears.
You can also use it to navigate through transients during playback.
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Version 3.2.3 Release Notes (June 9, 2016):
New features and improvements:
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Channel input monitoring meter can now again be activated with arm button only when Tape Style monitoring is off
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Edit Cursor is placed only when “Range Tool in upper event area” mode is activated and clicked in upper track area
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Disabled automatic loop activation
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On stop, only locate to edit (orange) cursor when "return to start on stop" is active
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Trim whole envelope with "bracket" tool beyond last / before first point
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Show absolute and relative values in automation transform tooltip
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