Per the Plan Builder Overview: (2nd paragraph bold and split out for emphasis, since that applies here.)
Step 5: Events
Add your goal events. These can be cycling events, triathlons, or even stage races. We recommend adding at least 1 “A” priority event for your season.
If you don’t have any events you are training for select “I’m Not Training For an Event”, and you can have a plan built to just make you faster in general.
Right now, it is an either/or proposition. If you choose the 2nd option, it is my understand that the PB will NOT update.
So, if you want to have the option of adding events in the future, you should add one as a place holder. Then you will be in the “Event” side of the option.