The Current Bar and Bar Offset

To clarify what the "current", or "most recently completed" bar is:

1. Look at a chart.
2. Make sure the chart is scrolled all the way to the right.
3. Point at the right-most bar. This bar is still being made. It's not complete.
4. Point at the bar just to the left of the right-most bar. That's the most recently completed bar.

ATM uses the most recently completed bar for its calculations.
 

Bar Offset

The "bar offset" is the number of bars before the current bar. So a bar offset of zero refers to the current bar itself (the default). A bar offset of 1 means the bar before the current bar. A bar offset of 2 means the bar before that. And so on.

When you create a rule and use a bar offset greater than zero, ATM steps back that number of bars and does its calculations as of the situation as it was back then.

The bar offset is most often used when looking for when one line crosses another. This event is often used for trading signals. To check for such a cross, 2 rules are needed: (1) check that line 1 was under line 2; and then (2) check that line 1 is now above line 2.

Rule (1) has to use a bar offset of 1 to check what the situation was 1 bar ago. Rule (2) uses the bar offset of zero to check what the current situation is.