I believe this tells the ship what weapon to use its engagement range for. So if a ship has a weapon with a range of 6000 and a range of 2000 a second weapon, the index referenced here will make the ship engage at either 6k or 2k.
Weapon 0 = 6k range
Weapon 1 = 2k range
m_weaponIndexForRange 1 = Ship moved up to 2k range to target before stopping its approach.
This is correct. In fact, the reason that everything else is set as index 0 is that pretty much everything besides SBs that have multiple weapon types have the same range on all weapons.
SBs have a very-long ranged weapon. Though, IIRC, the Advent SB is actually set for Index 1 as it's second weapon, and not it's third, is it's LR battery.