That's the value to supply for the
TargetType parameter when creating an AWS CodeStar notification when you want to target an AWS Chatbot Slack channel.
I don't know why, but this is not listed anywhere in their docs. In fact, a Google search for "AWSChatbotSlack" shows the only place on the entire internet this is mentioned was in an example response on this somewhat-but-not-really related docs page.
Edit: AWS has updated their docs to include this