The mid point price of the D1 candle determines where the upper and lower lines are drawn.
So the mid point of the closing candle on the 19/4/19 was1.6807 on AUD/NZD.
The upper limit is 30 pips up for a buy trade and 30 pips lower for the sell.
The hour chart is used to look at what the current trend is and confirming that the trend direction is correlating with the longer term trend at 1 day.