Build a bot from scratch allows you to build your bot from the ground up. If you select this option your playbook will only have one message node pre-populated in it and you can build out the rest of the flow from there.
You can access Build a bot from scratch by clicking Playbooks > Create chat playbook > Bot/Live Chat. You will then be prompted to select your audience for the playbook. After this step, scroll down until you see Build a bot from scratch.