All bingo machine have stepper units, and most of them do exactly what their name says (e.g. score disc, extra ball disc, magic square feature unit, etc).
One unit that is in most games is the selection feature unit, and while it tends to do different things in each game type, in general it's safe to say that it's job is to make sure things happen in the correct order where necessary.
An example on many games would be the extended/extra time/play function (move numbers before 4th/before 5th/after 5th), and associated rollover buttons. You always get the yellow rollover lit, then the red, then before 5th, then after 5th. Usually there is a trip relay associated with the rollover buttons, and since you can't untrip a trip relay, the selection feature unit does stuff like turning off the lamps for rollovers that are irrelevant if you already have the feature the rollover would enable.
Surf Club is actually a more interesting machine, so it's just your luck that you now get stuck reading a bunch of stuff that almost certainly won't apply to your game.
yeah, yeah...but what does it mean?OK, historically, the very first bingo had a "selector unit". It stepped up every time a coin/credit was played and enabled payout for each successive bingo cards (1-6). I suppose you could say it selected which cards were enabled (you probably wouldn't say it that way, but that's the way people talked in the early 1950's).
Bally pumped out a few machines, and then in 1952 Bally Beauty comes along. It had a knob on the front door which let the player choose which number from the set [19,20,21,22] to spot - if the feature was enabled. Bally called this the "Selection Feature" on the backglass.
The unit that kept track of which cards were enabled became the "Card Selection Unit", and a "Selection Feature Trip Relay" was born. It tripped when you got the selection/spot feature enabled.
Almost there....the very next machine was Beach Club, which really packaged up a lot of features from previous games. Of immediate interest is the spotting. Like Bally Beauty, this machine would let your spot from a set of four numbers. However, up to three additional numbers could be added to the set. It worked in a kindof interesting fashion. A set of four arrow lights would advance. The first three were driven by the selection feature unit, and the fourth by the selection feature trip relay.
The selection feature trip relay could trip at any time, and when it did at least the spotting from the set of four numbers was enabled. A circuit would automatically step the selection feature unit up to the fourth position if it wasn't already at least that far. If the selection feature unit was stepped up more positions, then the player magically got to spot from a bigger set.
In other words, the selection feature unit was visibly stepping up for the first three steps, then may be quietly stepping up further but the player doesn't know how much unless they get the selection feature trip relay to independently trip.
Anyway, the unit was named, and the name stuck for future machines even when the game didn't have the selection/spot number feature.
From a game play perspective, assuming the selection feature unit has stepped up to at least position 6 so it tripped the superline trip relay, you can use the knob on the front of the game to choose whether to spot number 9, 14, 15, or light three adjacent arrows in the superline. For example, if you choose to light superline arrows under numbers 11, 10, 20, and you hit holes 10, 20, and 5, you would get paid for a 4-in-line and you'd be wishing you shifted the arrows one more place to the right.
Here's the inevitable scan of the manual page.