Sort of. If EXTRADATA is set, the open or close price of a bar is the open or close price of its first or last tick. Otherwise it's the center price of its first or last tick.

Since the uncertainty range of a tick close price is bigger than the difference of center and close, a backtest without EXTRADATA is not less accurate. The remark under "News" is therefore a bit misleading. But you should set that flag when you use low resolution historical data, such as M1 data for one-minute bars, or D1 data for 1-day bars.