Text-based vs. Symbol-based
A text-based AAC system has an incorporated keyboard. This system suits people who type the words they want to say. They can also choose from pre-written phrases/words. They can read and write. Basically, text-based AAC is a combination of spelling and choosing pre-written messages.
Many AAC systems are symbol-based. The most majority of people needs symbols and pictures to communicate. These types of people usually can't read or spell. Having visual symbols that represent words or phrases helps them. However, some people know how to write and still find symbols more useful.