@B4chi,
Thanks for your inquiry. Please use the following code example to create the nested list. Hope this helps you.
Document doc = new Document();
DocumentBuilder builder = new DocumentBuilder(doc);
builder.ListFormat.List = doc.Lists.Add(ListTemplate.NumberDefault);
builder.ListFormat.ListLevel.Font.Bold = true;
builder.ListFormat.ListLevel.Font.Color = Color.Red;
builder.Writeln("Main Section1");
builder.ListFormat.ListIndent();
builder.ListFormat.ListLevel.Font.Italic = true;
builder.ListFormat.ListLevel.Font.Color = Color.Green;
builder.ListFormat.ListLevel.NumberStyle = NumberStyle.Arabic;
builder.ListFormat.ListLevel.NumberFormat = "\x0000.\x0001";
builder.Writeln("Sub Section1.1 ");
builder.ListFormat.ListIndent();
builder.ListFormat.ListLevel.Font.Underline = Underline.Dash;
builder.ListFormat.ListLevel.Font.Color = Color.Blue;
builder.ListFormat.ListLevel.NumberStyle = NumberStyle.Arabic;
builder.ListFormat.ListLevel.NumberFormat = "\x0000.\x0001.\x0002";
builder.Writeln("Sub Section 1.1.1");
builder.ListFormat.ListOutdent();
builder.Writeln("Sub Section 1.2 ");
builder.ListFormat.ListOutdent();
builder.Writeln("Main Section2");
builder.ListFormat.ListIndent();
builder.Writeln("Sub Section2.1");
builder.ListFormat.RemoveNumbers();
doc.Save(MyDir + "Out.docx");