Okay, thanks Andrew. I see. Anyway, it's a shame that I can't use the F10 shortcut to toggle the loop button on the fly, after selecting a region (section of measures) in the Chord Sheet.

Also, even though I can use the alternative of pressing L followed by the number of measures I want to loop and then Enter, it's not ideal because the active measure display often gets out of sync.

No, it's a real oversight and a crucial feature for a program like Biab. I demand that this Loop button become a toggle button that can be toggled on and off using the F10 shortcut on the fly, after selecting a region (section of measures) in the Chord Sheet.