The XML file format was introduced with Excel 2003 (XML support was introduced in Excel 2000). Excel 2007 has new XML file formats. The *.xlsx format represents a workbook that does not and cannot contain macros. The *.xlsm format represents a workbook that can or does contain macros. This separation is a security enhancement that lets the user know in advance of opening a document that there may be code inside. Remember the “This document may contain harmful code” warnings in previous Microsoft Office versions? With these, you would not be aware of the possibility of code until you opened the file. Now, your users will know in advance if they are opening a workbook containing code.