I do minimal wrenching and I know that on certain parts you want to mark where they line up; driveshaft for instance, so you install them back the same way. However I am not certain why there is an X here. Other than to say, don't install a bolt.

 

Probably just a mark the assembler puts it on it that shows he/she was there. They use optical sensors all thru out the assembly process to log certain steps so they know nothing has been missed.

Here's a fascinating documentary of our trucks being built.
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Thanks for that video! I really need to check out the factory tour in Dearborn, Mi.

 

Welcome, I've seen it at least 4 times, the video I mean, I'd like to do the plant tour my self but honey wants to go to Hawaii first.
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I think Rnl nailed it. Referred to as a "tick" mark, it's often used to confirm that a process or assembly has been completed. It's a method of prompting the line worker to be sure it's done by making them place the mark on the item.