I'm kind of interested in Infinity and want to start with the Japanese Sectoral Army from the Yu Jing faction and was wondering if they were in the normal rulebook, or just the expansion book.

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Drive me closer, I want to hit them with my sword.

Sectorial rules&lists were introduced in the expansion book Infinity: Human Sphere, but they refer to models' profiles&stats described in both books, so you'll need both even if you're just interested in sectorial armies.

The two table of contents are:

Core Rulebook:
- Background story
- Rules
- Skills
- Equipment
- Weapons
- Models' Profiles&Stats for all original Factions (Ariadna, Haqqislam, PanOceania, Yu Jing, Nomads, Combined Army, Mercenaries)

Expansion Book: (Human Sphere)
- Ongoing Background story
- New Rules (Linked Teams, Sectorial Armies)
- New Skills
- New Equipment
- New Weapons
- New Models' Profiles&Stats for all original Factions (Ariadna, Haqqislam, PanOceania, Yu Jing, Nomads, Combined Army, Mercenaries) + 1 new Faction (ALEPH)

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Marvin, the Paranoid Android: "The first ten million years were the worst. And the second ten million... they were the worst too. The third ten million I didn't enjoy at all. After that, I went into a bit of a decline."