It's just some wonkiness with initiative and things at less than 10,000 km that aren't firing at missiles in final fire. It was in VB6.
Basically, if you engaged the enemy at 10,000km, but didn't get to move first, the enemy would move away 9/10. This movement in VB6 did not provoke return fire unless you had a beam FCS with more than 10k km range. So depending on the speed of the enemy, according to vb6 rules, you might not get a shot off with your ship, but the enemy will... depending on your turn order.
20,000 km range was simply the range where no matter what, the enemy would provoke reaction fire. It had to do with weirdness in the per increment cycle. Missiles also had a thing where at 10k km or less pd wouldn't fire. That was fixed in C#. Unsure about the beam thing though.
However, IIRC, reaction has changed in C#.
Officers in VB6 had an "Initiative" rating. It influenced turn order.