I know what you're talking about and I was confused in the same way by the same guide
So here's my reply. The information in the British Expat guide is incorrect.
You can link an application to only one account. And yes,
it can be either sponsor's account or PA's one, it does not matter. You do need to use only PA's info though. Please don't be confused about it. Again, even if you're linking an application to sponsor's account, PA's info must be used.
Basically, the difference is only who will be getting emails about any updates. If a sponsor is more into the process and understands it better, then link it to sponsor's account.
If you did link it to one of them and now want to re-link it to another one, you still can do that. But as soon as an application is linked to a new account, an old one will show a message something like "An application has been linked to another account".
Please don't mix this up with ECAS. There it's a different story. You can use it with both sponsor's and PA's info. If you use sponsor's, you see both applications (yes, for CIC they are two separate applications). If you use PA's info, you'll only be able to see PA's application.
And the last thing. You don't have to have accounts at all. Even if you haven't linked an application to any account, you will still be getting all the required information, such as BR, MR, etc, via emails.
And finally
Even if you've linked an application to an account, you will still be getting some info in just emails (not via an account), such as AOR email, SA email, transferred to VO email, AOR2 email, and some additional docs request can also come as an email and not an account update. And these emails can be sent to both a sponsor and a PA, so please do check both mailboxes so that you don't miss any important request.
I hope this helps
