The ASL captioning allows for a live interpreter to sign along with or without the written captions.
As the SignUp website describes: “SignUp was founded because many young people from the Deaf community find written captions indescriptive, or even absent from media sites. A 2017 study showed that 80% of children who signed face challenges with reading – making written captions inaccessible. It will also be used as a learning tool for ASL students. SignUp is a fun way for kids (and adults!) to enjoy mainstream movies.”