From Halopedia, the Halo wiki
Universe and lore
Highlands is set in the Military Wilderness Training Preserve in the aptly named Highland Mountains on Reach. From 2517 to 2525, the SPARTAN-II program used this area for training the super soldiers on wilderness survival. Outside the map, the Big Horn River dominates the valley floor and in the distance, several CCS-class battlecruisers can be seen glassing Reach.
The middle of the map plays host to a D77-TC Pelican dropship, Bravo 001. During the Fall of Reach, members of SPARTAN-II Red Team deployed to the surface of Reach in Bravo 001 to defend the planet's orbital defense generators from Covenant assault. The Pelican was shot down and crash-landed in the Highland Mountains.
The following are locations on the map that are so named on the player's heads-up display during gameplay:
During development Highlands was known as Training Preserve. Current files on Bungie.net are even labelled as Training Preserve instead of the map's final name.