The persona is a device for personalizing the output of the software so that Sewers puts your letterhead and contact info in all the right places. It also stores your preferences and can be used to override some features of Sewers.
You can use multiple personas. One reason to use multiple personas is so you can design systems for someone else under their letterhead. Another is so that you can use different preferences for different agencies or clients.
You are required to define at least one persona, the “Default Persona”.
See also: Persona Editor