The base d20 resolution mechanic and the core combat/character formulas of Chroniques Oubliées (DEF, Attack, Damage, Hit Points, Initiative)

Recovery Points (PR) and the short rest mechanic used to regain Hit Points between fights

États préjudiciables — the detrimental conditions a creature can be affected by in combat

The two categories of protective equipment: worn body armor and shields

Properties defining how a weapon is used in combat

Categories of miscellaneous goods (adventuring gear, food, mounts, lodging, real estate)

Creature families of the Chroniques Oubliées bestiary, as grouped on co-drs.org

Broad weapon proficiency categories referenced by classes (in addition to individual weapon names)

Creature and race sizes

Source ruleset a race comes from

How a technique/ability is used in the action economy