It basically makes the map as you play in career mode change as the inner sphere changed with all mercenary units etc. moving as they do in canon. They've also changed the rules a bit to much more closely match the actual physical game. So let's say you read a book in a certain year and on a certain planet there was certain units with certain mechs. You go then and there and fight a mission against that unit they will be the correct unit and correct mechs.

What this does not do is anything beside tanks and mechs.
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BattleTech Extended is multple Expansions to the base game. Adding the universe of BattleTech in to the HBS game, with the official time line as written while also adding variably selectable tabletop inspired improvements to the game.
The whole Inner Sphere is represented, including the many factions, units and mercenaries populating the 'Sphere. Clans invade, ComStar and Word of Blake will be jerks, elite units may provide harder missions, the Bounty Hunter may hunt you.
'Mech and Vehicle rarity is based on Xotl’s 7 highly researched and well regarded D1000 tables and changes based on date, interpolating between the tables as days in game pass. Force compositions are the closest to what the universe of BattleTech is meant to look like possible with very little apocryphal content, other than nods to what has been seen in other games or fun parts of the lore like the Crescent Hawks.
The time in game will change the map, the events that are occurring, the units that are garrisoning planets, the technology available in different areas of space, the 'Mechs and 'Vees that the Op For is using. All based on source books and novels.