When you use a text-based chord chart in OnSong, you have a lot of options on how that chord chart looks and behaves. One of these ways to is view the chords in various numbering systems.
To do this, view a chord chart with chords that are detected and then open the Style Preferences Menu by tapping on the sliders icon in the Menubar. Next, choose the Chords tab at the bottom if it's not already selected. This gives you tools to transpose chords and format them in various ways. Locate the Style section at the top and tap on one of the following styles to view the chords differently in the Song Viewer.
This the default view of chords which uses the capital letters A through G including sharps and flats for the designated key. You can adjust how chords are outputted by making changes in Settings > Display Settings > Song Formatting > Chords or by tapping on the wrench icon in the upper left corner of the Chords tab.
This number system uses the numbers 1 through 7 to represent the position of the chords relative to the root of the key. For instance, the "1" chord in the key of C would be C major, the "2" would be D minor, the "3" E minor, the "4" F major, the "5" G major, the "6" A minor and the "7" B diminished. Nashville often uses symbols to replace the chord characteristics. You can turn on Use Symbol Replacements to enable that aspect of Nashville Notation. You can also enable the Use Superscript option as well.
This method is common for students of music theory and uses roman numerals to express the relative position of the chord to the key. In the key of C using this style, "I" is C major, "II" is D minor, "III" is E minor, "IV" is F major, "V" is G minor, "VI" is A minor and "VII" is B diminished. One advantage of this system is that aside from the root, any chord that contains and "I" is minor. You can also opt to use Lowercase Minor Chordswhich is a typical style used with Roman numerals to quickly learn the structure of a chart.
Lastly, Solfege is a fancy way to represent chords like in the famous The Sound of Music song, "Do Re Mi". This style is particularly well-known in many European countries. OnSong will display the following to represent the position of the chord in the key. For instance, in the key of C, the "do" would be C major, "re" would be D minor, "mi" would be. E minor, "fa" would be F major, "so" would be G major, "la" would be A minor, "ti" would be B diminished. One option that Solfege has is a concept called "movable do". This is the default where the "do" moves as the root of the key so that if you change key, the root will always be "do". You can changed this to "Fixed Do" which locks "do" to always represent "C". You can adjust this in Settings > Menu Settings > Style Preferences > Chords > Key > Solfege Type.
If you want to apply this to a book or your entire library you can do this by opening the Apply Styles Menu by tapping on the three dot in the upper right corner of the Style Preferences Menu. Then select the book, set, or all versions. Depending on how larger your library is, this could take a few seconds to complete.