The Source Text of the Text Generator is a plain text string, editable in the layer Stack, Properties panel, or using the Text Tool in the viewport.
The Source Text parameter contains plain text only, meaning that it does not define the format of the text (color, font, etc.) on the screen: this is defined through separate parameters under the Default Style group, or with Animators.
As with image layers, is it possible to add Generators and Modifiers to the source, except they will generate and modify text strings instead of images.
Using the Timecode To String Generator bound to the Source Text, it is possible to render a dynamic timecode on top of the composition.