A sub-page in a process can have only one start stage, which is where the execution begins when the sub-page is called from another page. A sub-page cannot have zero or more than one start stages, as that would cause ambiguity or confusion in the execution flow. References:
https://portal.blueprism.com/sites/default/files/2020-06/Blue%20Prism%20Certification%20-%20Developer.pdf (page 2)