Usually it is a free replay but some machines let you go into test mode and change it to a free ball. I do that on mine since it is set on free play anyway.

I set mine in the menu to an additional ball as well. When a machine is set to free play it just seems lackluster to get the Special and then get nothing for it. At least at an arcade you usually get a free game.

Back in the 70's when I was playing pinball in my youth lighting and getting the "special"
was pretty special, because when you're playing with a limited amount of quarters like you only had 25 or 50 cents and you popped a free game it was a great accomplishment.

It meant that you were a pretty good player and anyone watching you hit the special were
impressed, unless they were a much better player than you and they couldn't wait till it was
their turn to get on the pin.

When I was 13-14 years old my family had a membership at a local swimming club. They had a pinball machine there and would watch the older guy's that would play it. I was always amazed at how some of the better players could stay on a pin for a long time because they would get replays and just kept adding them onto the machine. I might have wanted to play it, but there was no way that was going to happen when a guy was ON and wasn't leaving till all those credits he had used up.

On my old gottlieb you can set it to give a Bonus score plus 1 credit, or an extra ball. The only draw back to the extra ball setting is that on mine you can only earn 1 extra ball "per/ball in play" which means if you had already earned an extra ball you wouldn't get another. The bonus is pretty hefty for the era (100,000) so i usualy keep iot on bonus and credit.

I set mine to an additional score for home play.

The word special is used because the award can be adjusted by the operator. Most of the time it awards a credit but on some games it can be adjusted to award an extra ball or points. In certain areas it used to be illegal to award free games so it would be changed to extra balls. If the word REPLAY was printed on the playfield it would be weird if the game was adjusted to award another ball.