In Excel 2007, cells can have pop-up comments that provide notes to the reader. These comments are easy to add using Excel 2007 itself. Reading the text associated with comments from the XML package is easy. However, inserting comments into a spreadsheet programmatically
is very problematic. To add a comment you need to write the text into the /xl/comments1.xml file and instructions on how to draw the comment into the /xl/drawings/vmlDrawing1.vml file. Yes, you got it
draw the comment. VML stands for Vector Markup Language and tells Excel how to present the comment. VML is a
legacy format, support for which crept into the ECMA Open XML standard at the last minute. If anyone would like to try their hand at writing the required VML code, then please join the ExcelPackage development team to add this functionality.