Documents in Reflow can be composed of one or several scores.
Every score is a different representation of your song, and is given its own style and engraving rules.
Using multiple scores is not mandatory, you could simply use just one score and modify all its settings when needed.
However, one common usage would be to have a Full Score with all the instruments present, set in concert tone (i.e. non transposing). And one score for every single instrument, set in transposing tone, hence effectively creating a full score and individual parts setup for your document.
You can bring up the Score list and settings by tapping on this row of the Settings dialog.
This will bring up the Score list, where you can create or remove scores.
To open the score properties, tap on the (i) button.
Lots of options are available on this page, affecting the visual representation and layout of your score.
One of the more important options here is probably the Bars per system row, this allows you to increase or decrease the number of bars every system will have by default.
The measures per system are fixed by default, but you can force a system break at a specific measure using this keyboard button.