I don’t have the rules in front of me, but there is a vague mention of “corps” in the rule book. And while not explicitly mentioned, that is probably the best approximation.
As state above, it’s not set in stone. And if anything, it represents a typical European corps. But that in turn was equivalent to an American or Bulgarian division. (US corps were larger; Bulgarians had no corps. Only field armies and very large divisions).
The higher, starting concentration of artillery simulates the per gun total per division, I think. German artillery was better suited for high trajectory fire. Meanwhile, the typical amount of artillery per Turkish division was about 25 guns (Hence 1 artillery piece with most Turk unit concentrations).
If you house ruled this, you could use different attack/defense values for each nation, esp. on a D12.