Shock infantry

Light infantry refers to two different types of units depending on whether referring to Age of Empires III or the expansions The WarChiefs and The Asian Dynasties.

Age of Empires III

In Age of Empires III, all ranged infantry units that counter heavy infantry and light cavalry with their ranged attacks are considered light infantry. There is no unit in Age of Empires III that has a direct attack bonus against light infantry, but they are still easily countered by artillery and melee cavalry (heavy), and some heavy infantry in melee mode. That is because light infantry units share a similar set of statistic values such as low hit points, low hand attack and ranged resistance, which makes them vulnerable to melee and siege units. The exception being the Cherokee Rifleman. Examples of light infantry units in Age of Empires III are:

The WarChiefs

In the first expansion, the definition of light infantry changes drastically. In The WarChiefs, Light Infantry constitutes a small set of foot units that emulate cavalry (which Aztecs do not have, for historical reasons) thanks to their high speed and ranged resistance. Therefore, light infantry are countered by heavy infantry and are good against artillery.

The units formerly tagged as light infantry are no longer considered as such and do not possess the tag anymore, being tagged as just Infantry and Ranged Infantry; they're differentiated from the Musketeer-type of units by the Heavy Infantry tag they do not have.

The only light infantry units in the first expansion are:

Coyote Runners are a substitute unit type for hand cavalry, so they are also countered by ranged cavalry. Eagle Runner Knights are a substitute unit type for ranged cavalry, so they are also countered by long-range infantry.

The Asian Dynasties

The second expansion continues to use the light infantry definition from The WarChiefs, and adds three new melee units.

The complete list of light infantry units in The Asian Dynasties is:

Coyote Runner
Eagle Runner Knight
Tiger Claw (unique among the Light Infantry units, Tiger Claws have the Infantry tag)
Rattan Shield
Disciple (tagged as a Coyote Runner by the game, and therefore vulnerable to ranged cavalry).

Definitive Edition

Age of Empires III: Definitive Edition renamed light infantry to shock infantry, to end the confusion that some players made between Skirmisher-type and Archer-type units, and units like the Coyote Runner.

The complete list of shock infantry units in the Definitive Edition is:

Coyote Runner
Eagle Runner Knight
Tiger Claw (unique among the Shock Infantry units, Tiger Claws have the Infantry tag)
Rattan Shield
Disciple (tagged as a Coyote Runner by the game, and therefore vulnerable to ranged cavalry).
Chasqui (tagged as a Coyote Runner by the game, and therefore vulnerable to ranged cavalry).
Chimu Runner (tagged as a Coyote Runner by the game, and therefore vulnerable to ranged cavalry).
Shotel Warrior (tagged as a Coyote Runner by the game, and therefore vulnerable to ranged cavalry).

Leftover Units

Additionally, because of an apparent oversight from the developers, in The WarChiefs, The Asian Dynasties, and the Definitive Edition, some units keep their Light Infantry tag from Age of Empires III, making the new multipliers apply against them.

This oversight is probably due to them appearing only in the campaign (and in treasures for the Aztec Eagle Warrior).