Flat allows you to display Kodály notation, that is generated directly from the content of your score.
To enable Kodály notation, you need to open the instrument settings. You can do so by clicking on the instrument button on the left on the toolbar, then click on "Manage instruments". Or you can click on the instrument name on the score.
From there, you can configurate the Kodály notation:
You can choose to display notes as letters (do re mi fa so la ti), or as hand signs.
You have the option to use either a Fixed Do or a Movable Do system.
If you choose Fixed Do, the solfege notation will directly correspond to the actual pitches you sing. In this system, a "C" will be referred to as "Do".
On the other hand, if you opt for Movable Do, the solfege notation will be determined by the key you are in. In this case, the tonic note of the key will be called "Do". For example, if you are singing in the key of "G", the "G" note will be called "Do", and the "A" note will be called "Re".
The source singing pitch refers to the "original" pitch of the song, while the recommended singing pitch is the pitch that students are advised to use when singing the song.
If you're using a Fixed Do system, the Kodály notation will be automatically transposed according to both the source and recommended singing pitches.
In this specific example, the source singing pitch is "C" and the recommended singing pitch is "G":
When printing your score, you might want to keep only the Kodály notation and remove the regular staff. To do so, you just need to enable the option to "Only print Kodály".
Here is the result, with only the Kodály notation: