"Once per bar" knows which is the current bar and this block can only pass 1 time per bar. When it runs under "on Tick" and there is no block above, then "Once per bar" passes in the first tick of each new bar (or candle).
If the block is under some "Condition" and that "Condition" passes somewhere in the middle of the bar, then "Once per bar" will pass at that time, let's say at 20th pip in that candle.
There are "Candle ID" parameters in many tools in "Condition". The EA always knows the OHLC values of the visible candles on the chart, so when you change "Candle ID" you can get O, H, L or C from the 20th candle, or from the 35th, or from the candle you want. The same is true for the indicators. You can get indicator's value from the 3rd candle or from any particular candle just by changing "Candle ID"
