Example:

Document doc = new Document();
DocumentBuilder builder = new DocumentBuilder(doc);

// Each paragraph has an OutlineLevel, which could be any number from 1 to 9, or at the default "BodyText" value.
// Setting the property to one of the numbered values will show an arrow to the left
// of the beginning of the paragraph.
builder.getParagraphFormat().setOutlineLevel(OutlineLevel.LEVEL_1);
builder.writeln("Paragraph outline level 1.");

// Level 1 is the topmost level. If there is a paragraph with a lower level below a paragraph with a higher level,
// collapsing the higher-level paragraph will collapse the lower level paragraph.
builder.getParagraphFormat().setOutlineLevel(OutlineLevel.LEVEL_2);
builder.writeln("Paragraph outline level 2.");

// Two paragraphs of the same level will not collapse each other,
// and the arrows do not collapse the paragraphs they point to.
builder.getParagraphFormat().setOutlineLevel(OutlineLevel.LEVEL_3);
builder.writeln("Paragraph outline level 3.");
builder.writeln("Paragraph outline level 3.");

// The default "BodyText" value is the lowest, which a paragraph of any level can collapse.
builder.getParagraphFormat().setOutlineLevel(OutlineLevel.BODY_TEXT);
builder.writeln("Paragraph at main text level.");

doc.save(getArtifactsDir() + "ParagraphFormat.ParagraphOutlineLevel.docx");