I would take the term literally and say that "additional needs" means that you can do everything the same as a normal person, but have needs that others don't have, and "special needs" means that you are unable to do something the same as a normal person, and so need the task to be adapted so you can do it.
For example: I would consider my need for a second desk in elementary school to help with my organization problem, an additional need, because it didn't change the way I did my work, or the way people taught me, it was just an extra thing I needed to succeed.
By comparison, my need for extra time on my classwork and exams, as well as receiving truncated homework with fewer problems, I would consider a special need, because I was unable to complete my work in the same way as my classmates, so I needed those tasks adapted to me so I could do them.
If that makes any sense.
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"Survival is insufficient" - Seven of Nine
Diagnosed with ASD level 1 on the 10th of April, 2014
Rediagnosed with ASD level 2 on the 4th of May, 2019
Thanks to Olympiadis for my fantastic avatar!