A unit from the Troops, Heavy Support or Fast Attack sections of the army list with 3,000 or more experience points becomes Elite. These specialised troops are often called upon to fight in other battles and their availability is limited. To represent this, Troops, Fast Attack and Heavy Support units with 3,000+ experience count as Elite troops for choosing the army, deployment and so on in subsequent battles.
Becoming Elite
Source: Core Rulebook 3E, p. 161
Troops, Fast Attack, or Heavy Support units with 3,000 or more experience points count as Elite troops for army selection and deployment in subsequent battles.