I'm a beginner piano student and have come across a type of chord notation I'm not familiar with, and can't find anything on the internet.
Below is a photo of part of the chord chart for the worship song 'A thousand Hallelujah's by Brooke Ligertwood.
I believe the superscript numbers mean 'add the 2nd degree of the tonic scale' - so G♭² is a G♭ major chord with the 2nd degree of the G♭ scale added. That is, Gb - Ab - Bb - Db.
What does it mean when a superscript number has brackets around it though? E.g. A♭⁽⁴⁾?