From the rules' introduction:
Hallowed Ground has been written and expanded to cover ground warfare from 1836 to 1878. In essence, Hallowed Ground covers conflicts from the Texas War of Independence through the Russo-Turkish War. With this supplement, gamers can play periods including the Mexican American War, Crimean War, Indian Mutiny, Great Paraguayan war, American Civil War, Austro-Prussian war of 1866, Franco-Prussian War, and the Russo-Turkish War.
This greatly expanded version of Hallowed Ground introduces two new game scales into the supplement: Company scale and Grand Tactical scale. The three game scales are necessary for the period covered by Hallowed Ground due to the extremely varied types of battles and engagements possible over the 40 odd years encompassed in the supplement. A game of a single scale cannot be expected to handle everything from a few companies of British infantry skirmishing with mutineers during the Indian Mutiny to the multiple Corps battles of the American Civil war and Franco-Prussian war. Additionally, the three game scales provide an opportunity for the gamer to experiment with games of different scopes with his existing troops and mounting systems. All three scales use the same basing system; there is no need for rebasing.