When there is no need to schedule a Budget Line because e.g. it is an outsourced service or expense, etc., you can use the "Forward Budget Details To Actuals" option. Click on the Budget Line to highlight it in yellow, right-click on it, and select "Forward Budget Details To Actuals" from the Budget Details menu. This will create a Project Object on the Actuals Project View with the same price (buy and sell) and quantity, and at the same time, it will get attached to the Budget Line.
When using this option on a Budget Detail that already has one or more Actuals attached to it, there is a warning dialog "An Actual Is Already Attached To This Budget Detail. Are You Sure?". This is to avoid creating duplicate Actuals by mistake. But you can still click Confirm to get the Actual created again.
If you need to reverse this process and delete that Actual, you can click on the Actual line to highlight it in yellow, next right-click on it, and from the Actuals menu select "Delete Actuals".
Note that this is different from "Detach Actuals" which only detaches the Actual from the Budget Line but does not delete the Actual.