Hi Craig,
Just to let you know that the mystery is sorted out. "Booking restricted" status refers to the fact that there is a condition/ some prerequisites set for being eligible to taking up the event. In the set up it is called "Restrict Access" and the restriction relates to a competency level.
As an admin, I need to be description sensitive, so I did not assume restrict access related to competencies at one place will be called booking restricted at another place. To me, booking restricted would mean that people cannot enroll even though the places are available. For example, booking restricted by competency would not be confusing.
Anyway, thank you for your time and effort.
Anna