Once your account has two or more guidebooks, each one has its own Use guidebook defaults toggle, on by default, so it inherits your account settings. With a single guidebook the toggle doesn’t appear. The Use guidebook defaults toggle saves as soon as you switch it.

While a guidebook is using the defaults, its Guest Registration controls are locked. Turn Use guidebook defaults off to edit them: the first time you do this, the guidebook takes a copy of your account settings and keeps its own configuration from then on, so later changes to the defaults won't reach it.

If you turned Use guidebook defaults off for a guidebook before, turning it off again brings back that guidebook's previous own settings instead of a fresh copy of your account settings.

Turning Use guidebook defaults back on re-applies your account configuration to that guidebook.

There's no Use guidebook defaults toggle on the defaults page itself — that page holds the account configuration itself.