The narrative relationship allows text be an integral part of the overall message of the comic, and it is used for dialogue, sound effects, titles, narrations,etc.
On this page, the sound effect text compliments what is being said in the caption boxes. As Tim-21 fixes the robot dog, Bandit, the sound effects compliment the change, and Bandit goes from saying "Fra!" to "ARF!" This is one example of text enhancing the narrative beyond what is said in the dialogue or narrative boxes.
Metanarrative relationships influence the structure of text in a comic, such as the layout of text boxes, and the specific fonts chosen for specific characters or purposes. Essentially, the metanarrative tells you about the narrative without being directly involved in the words.
In this two page spread, there is no uniform layout, and it is easy to get confused about what order to read in. That's what the text is for; as you can see, the text leads to read first left to right, then right to left because the text boxes make up a curve. Just think of it as connect the dots. You don't want to have lines running all over the place, so your eye follows the smoothest path, which is the curve of the text boxes.The text guides your eye over all the images in the two page spread, and ensures that you can make sense of the numerous, chaotic images.
The examples I have discussed are only small portions of the lettering usage in Descender, and lettering is a very well used element of the comic as a whole.