In reality, a ship would need to manoeuvre into contact and zero velocity relative to the other ship in order to dock with or board it.
They might well be hurtling away at millions of miles per hour in relation to anything else in space, but relative to one another, they must be at standstill, and — obviously — in physical contact.
So, how does ALIEN's abstract range system represent this?
I can only surmise that it means pretty much what it says on p.193 under "Dock" — that both need to be in the same range zone band on the space combat map AND have the same "approach velocity"; i.e. be moving along the space combat map in the same direction at the same rate.
That "approach velocity" actually seems to be a bit of a misnomer; both ships will of course always automatically have the same approach velocity relative to one another — if ship A approaches ship B at 120km/s, then by definition, ship B also approaches ship A at 120km/s. If A burns to reduce that velocity to 110km/s, B won't continue to approach at 120; any changes one ship creates in it will obviously affect the velocity. It is the same for both.
By definition, boarding can only take place at approach velocity zero.
I suppose they mean some kind of "objective velocity...?"
And there is really no such thing as "deceleration"; you only point your engines in one direction or another and accelerate ... along your current velocity vector to speed up, against your current velocity vector to slow down, or in some other direction altogether, to alter course — or, in the case of ALIEN's one-dimensional system, towards the other ship or away from the other ship.
But yes — again, looking at reality, if your two ships initially approach one another at 120km/s, then they would pass one another, like knights jousting. For one ship to overtake the other after that point, it would need to work hard.
So, the overtaking must actually begin already before they meet. If ship A wants to board ship B, they would flip and burn as soon as they detected the other ship, so that they, by the time ship B reached their position (with adjustments because B of course would burn hard in some suitable direction as soon as they could identify A's intentions), they'd have killed off as much as possible of that approach velocity. If they fail to match B's velocity vector, or fail to get into its vicinity when they do, they'll need to start chasing it instead. But that means that they need to travel faster than B to overtake it — but then they must have burned off that overtake velocity just as they reach B so that they at that point are at relative standstill.
If you're not somewhere that orbital mechanics interfere, it's a bit like trying to manoeuvre hovercraft on ice, except a lot trickier.
So, in game terms ... if the ships detect one another at extreme ends of the space combat map, and both travel at two zones per turn toward the opposite edge (in what the game calls "approach velocity" — but making the actual approach velocity four zones per turn), A will need to accelerate in the opposite direction ("decelerate") in order kill that velocity to try to match B's, (i.e. first reduce the two per turn to zero, and then back up to two per turn — or more — in the other direction) while B will try to speed up, in order to make this more difficult for A.
If A manages to land in the same column as B AND have the same velocity in squares per turn in the same direction, at the same time, he will have succeeded in manoeuvring into a position to try to breach.
As for a ship, irrespective of armament and gunnery stations, only being able to fire once per turn ... it of course makes no sense at all, from an objectively realistic point of view.
I therefore suspect it's purely a play balance issue — space combat is deadly enough as-is.
Before clicking that response button — are you sure you actually read it?