When option fill is on (f+), your lines will be broken on word boundaries based upon your current right margin. If at some time in the future you would like to change your right margin and refill your text you can accomplish this using the F4 function key. It will take the line your cursor is on and all following lines up to:
whichever occurs first.
A line may be marked as a paragraph start by typing an Alt–p. A paragraph symbol will appear at the end of the line. The Alt–p acts as a toggle allowing you to mark/unmark a line. Filling will stop at each paragraph start, that line will be indented and filling will then continue until one of the above two conditions is reached. The paragraph start is provided as a means to stop filling between two consecutive lines.
If a group of lines have been tagged as described under the F7 key, then only those tagged lines will be filled. Filling will step over each paragraph stop as described above.
Lines are filled between your left and right margins. If option justify ( toggle with Alt–j) is on (j+), then the text will also be right justified.