If you mean the highest discipline regiment is is displayed as the side's discipline during the battle, I'm not sure that's actually correct. laying as Prussia and having the overwhelmingly highest discipline by 10-15%, it always displayed the discipline of the leading general. At least I presume so since the only time I ever saw the shown value change was when another general took command. This was regardless of whether my troops were participating in the battle, even in cases where they made up over 80% of the troops currently fighting on the field (and/or waiting to enter the field). Hence my previous assumption that discipline -- at least the value it displays for the battle -- is tied to general.
That being said, anyone recall a source to confirm that discipline is indeed calculated on a per-unit basis? I only ask because since the battle sides' discipline is tied to general, and mousing over the individual units does not display any values for discipline, and the wiki/manual doesn't specify this either...