A live text uses text tags to display data. You can add all kinds of tags and it is not limited to one data source.
The live text watch part allows you to add 'live text', using text tags. When changing the text on a text layer you will have some extra options in the text dialog. You can add all kinds of text tags to add dynamic text to the part.
Just tap one of the tags, and it will be added to the input text. You can also write the tags in the text using the keyboard.
Live texts can be automated as well. The same tags can be used to position and transform your layers, so your can create custom automated watch parts.
Read more about automation in the automation section of the watch part designer manual