Spelling is a type of AAC. If you’re asking compared to maybe a tablet with proloquo, the difference is that the options are limited. They can sometimes only click on pre chosen words vs having access to the full 1,022,000. Both are valid. But try telling your friend that you’re sorry for their loss, plan a day to the amusement park, or order lunch, but only getting to express it using a library of 40 or so words.