What are you supposed to put in the additional information section?

The reason why you took multiple languages?
A picture of yourself so your application can have a face?
A thank you?
A resume to flesh out your extracurriculars?

These are all things I’ve heard people do, but I’m not sure which exactly is the correct thing to put. I would love some advice, thanks.

Only use the additional information section for things that need to be explained, not things like thank yous or images. For example, 150 words wasn’t enough for me to explain my involvement in one of my activities, so I briefly elaborated in the additional information section. It can be used for a lot of different things depending on your case and your application, but it certainly does not need to be used.

It is not for extra essays, pictures, thank yous, or resumes. It is for BRIEF explanations of extenuating circumstances that may affect you application. Or my kids did put some bullet points about a couple of ECs where they held a lot of roles or won many competitions (very concise bullet points).

Use it for: anything you need to explain, or the ability to expand upon an activity description when there are not enough character spaces in which to describe it adequately in the activities section.

Nothing that meets those criteria? Feel free to leave it blank! Sometimes, “less is more.” Admissions officers are busy people. Add stuff that enhances your application. Do not add anything that does not.

My son wrote nothing in that section.
There was nothing he needed to explain.
He was able to sum up his activities, etc., within the word limits in their own section, so he did not need to write anything in the additional information section.
He did not attach a resume or other materials either.
He got into his top choice, Williams, early decision, so it does not seem to have hurt him!

What would an extranuating circumstance be?

Okay, so I took a language outside of school, but my school put it on their transcript AFTER getting the transcript from the institution I take the language at. Would that be something to mention or should I leave it blank since there isn’t anything I feel like I need to explain?

Leave it blank then.

As others have said, just because the section exists does not mean that you need to fill it out. I agree with @TheGreyKing ; sometimes less is more.

I would further say that an extenuating circumstance is not to explain a bad grade or a bad test score or why you took (or did not take) a certain class. Such explanations (if needed) should be covered by the GC.

I shall leave it blank as originally intended before doubting myself then, many thanks!