The client surface is a login, not a permission level
Give a client access in Flowboard and they get their own login, by magic link or password, against a separate portal-users identity table rather than a seat in your workspace. Task, comment, and file visibility is scoped on the client surface, and the Preview as Client toggle filters your own screen using the same visibility rule the live portal uses, so the preview is a simulation rather than a mock. So what: you check what the client sees before you send it, instead of spending the afternoon after granting access working out what else they can now reach.