When you add a new translation, the original text you are translating is displayed in the edit form as the <em>source</em>. If at least one translation of the original content already exists when you add a new translation, you can choose either the original content (default) or one of the other translations as the source, using the select list in the Source language section. After saving the translation, the chosen source language is then listed on the Translate tab of the content.