X is for the unknown.
As a writer my favourite viewpoint is Third Person Omniscience. It means that I know everything about my characters from thoughts to feelings and am able to swap from one character to another (avoiding Head Hopping).
There is also Third Person Limited Narrative. This is when you tell the story from the vantage point of one character but they do not know the thoughts and feelings of other characters.
First Person relies on what your character knows and sees. They are unable to look into people’s heads and find out what they are thinking or feeling, and in addition they cannot know people’s backstories or life histories unless they have been told them.
I labelled this X the Unknown because even writing from Third Person Omniscience there are still unknown factors to your character or storytelling. When you are writing it from a certain viewpoint for example your character won’t know they are blushing unless they are looking in a mirror. They can feel heat radiating from their chest, feel it travelling into their cheeks and know they feel hot but without seeing a reflection how can they tell if their cheeks are red and flushed?
X the unknown is also for the vast array of things in the universe we simply don’t have an explanation for. All the unsolved mysteries. We are naturally curious and while we strive to have answers for everything there are things we just don’t know. Why did the dinosaurs become extinct? Was there a meteor that changed the Earth’s climate? What happened aboard the Mary Celeste? Why did they build the pyramids? What happened to the Mayans?
I admit to skipping ahead in a book to finding out the ending. The big reveal where the murderer is announced because I just couldn’t wait another moment to find out. Are you like that?