@JamesK wrote a fantastic answer for this specific term, but for generalization I wanted to add a couple similar terms that may aid understanding.
"Tarmac" is indeed how you defined it in the question -- a large, paved asphalt space which could have many uses, but in this case is used for standing room for the event. This is (in my experience) an uncommon usage of the term tarmac, which is far more common on airports, military bases, and other utility/transportation spaces.
In entertainment and ticket sales, the far more common term in American English would be a "General Admission" (or GA) ticket, which grants entry to the event, but not an assigned seat. This can take many forms, sometimes directly in front of the stage, but often this area will be somewhat further away from the performance with worse visibility than the assigned seating. Some outdoor venues will call this "Lawn Seating", if there is a grass field near the stage instead of paved tarmac. In the lawn case guests are often allowed to bring folding chairs or blankets of their own, but this depends on the venue.
All of these types of tickets will allow access to purchase food, drinks, and other merchandise vendors if they are present at the event. Just not an assigned seat for the actual performance.