The text is split into segments automatically, and sometimes you might need to join or split them.
To join adjacent segments, hold down the Shift or Ctrl key and select them with your mouse. Then press the button in the toolbar.
The segments will be joined.
It is possible to merge unlimited number of segments at once as long as the chosen segments belong to one paragraph in the source file and your merge segments one-by-one. If you get an error message, it means you are trying to merge segments that belong to different paragraphs.
To split a segment, place the cursor in the Source where you want the segment split, then press the button in the toolbar. The segment will be split into two parts at that point.