When a new version of Word is released, it invariable has new features. These features, being new, are not available in previous releases of Word. That means if you create a document in Word 2013, for example, and use some of the new features not available in Word 2010, those new features won’t be seen by someone who opens your document with Word 2010.
Compatibility mode in Word 2013 is there to help prevent you using features that are not available to users of older versions of Word. For example, if in Word 2013 you open a document that was created in Word 2003 (that has the .doc extension), Word will display the text [Compatibility Mode] after the document’s title.
This message tells you that all the features that are present in Word 2013 but not available in Word 2003 have been disabled. This prevents you from adding impressive new content that other readers might not be able to see.
If you want to make a change to a document created in a previous version of Word, and you know that everyone who might read the document is using the same version of Word as you, you may want to convert the document to make those attractive new features available. Click on the File tab to go to Backstage View, go to the Info tab and then click the Convert button.
The warning message that is then displayed tells us that the conversion may cause some minor changes in the document’s layout and the file size may actually be reduced. Performing the conversion replaces the old .doc file with the new .docx file.