Opening booster packs of a given expansion earns you pack points for that expansion. Those points can be exchanged only for cards that appear in that expansion.
Expansions are sets of cards that are labeled with a single code. The expansion code (such as A1, A1a and A2) tells you which cards belong to which booster packs.
For example, the cards included in Genetic Apex: Mewtwo, Genetic Apex: Charizard, and Genetic Apex: Pikachu have the A1 expansion code.
Note: Some cards that are not obtained from booster packs may be included in an expansion.