If a charge is declared against a unit with missile weapons it can respond by shooting at the charging unit but only if the charging unit is more than half its charge distance away. This reaction is called stand & shoot. The shots are worked out once all charges have been declared and before the chargers are moved. Work out the shots at the measured range and apply the -1 to hit modifier for shooting at a charging target.
A unit which stands & shoots does so out of the normal turn sequence, in the other side's Movement phase before chargers are moved. Note that chargers who begin their move within half their charge distance of the enemy cannot be shot at in this way. They are too close, and there is insufficient time for the unit to shoot at their enemy.
If the chargers start their charge beyond the maximum range of the shooting unit's missile weapons, the shots are made at the maximum range of the weapons. The archers let loose as soon as their enemies are within range.