What do the up and right pointing arrows mean?
A good model is read top to bottom and left to right. A formula should only refer to cells above it on the same sheet, or to cells on sheets to the left.
Where Openbox detects that a calculation breaches this rule, it will show arrows in the bottom left of a calculation block. Move your mouse over the arrow to show a message explaining the issue.
This is only advisory and will not prevent the model being built.