There is no clear indication of what the die roll is used for.
I haven't been playing RPGs long enough to remember any edition of Chivalry and Sorcery when it was new, so when I approached researching this question, it was with fresh eyes and no inherent assumptions about how combat in this game is supposed to work. What I've found is just based on the rules text, and some further research on Chivalry and Sorcery sites across the internet.
There is no mention in section 9.01 CLOSE COMBAT about what you are supposed to do after you "modify the 1d6 result". There is mention of minimum and maximum casualty numbers, and what to do when one side loses more figures than the other, but no mention anywhere of exactly how many figures you are supposed to kill based on your roll.
On other sections of the rules, there is explicit mention of how kills are calculated. For example, in section 8.01 MISSILE FIRE, it states:
A target figure is killed and removed from play when the fire team rolls 1d6 and equals or exceeds the die values given in the table below.
Other sections describe additional tactics that can be used to adjust the melee combat roll:
In any turn in which a 'lift of pikes' cannot be accomplished, the pikemen melee with -2 to their 1d6 combat rolls
And instructions for how to divy up casualties:
Once the total number of casualties suffered has been determined at the conclusion of the battle,
But in a fairly thorough perusal of the mass combat rules presented in that book, I can't find any indication of exactly how mass combat casualties are calculated.
Looking through some forums related to Chivalry and Sorcery, the overall trend that I can see is that people who love the system overall find the mass combat rules hard to understand and opaque. Given that there is no mention in the book in any relevant section about how to calculate losses in combat, and the dearth of information elsewhere on the internet, the only thing that I can conclude is that there are no rules written for what that die roll is supposed to be used for.