When you select a default file format, the way you use Microsoft Office and how you share files with people using other productivity applications can be affected. File formats were initially developed to support the features and behaviors of applications. For example, OpenDocument Format (ODF) was originally developed to support the features and behavior of OpenOffice-based productivity suites and OpenXML was originally developed to support the features and behavior of Microsoft Office.
Microsoft Office now provides support for ODF and gives people using Microsoft Office the chance to save their Office files in ODF. However, because Microsoft Office and OpenOffice features aren’t exactly the same or aren’t implemented the same way, people who use unsupported features (see the differences links below) and save Office files in ODF might experience changes in how they can work with the file and sometimes see a loss of content.