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 settings dialog using the Score > Scores menu.
The measures per system are fixed by default, but you can force a system break at a specific measure using this palette button.
This is called a system break. You can also press Enter on your keyboard to create/remove a system break.
Staff spacers are special symbols that can be added to your score and tied to a beat.
When attached to a beat, they increase the spacing between staves that this beat need.
With your cursor on a staff, call the Scores > Add Staff Spacer Above or Below menu to create one of these symbols.
You can then select them, move them and grab the handle on the bottom side (or upper side) to resize.
To remove a spacer, select it and hit the Delete key.