Core

"Core" is the term that is being used to label all game content that is not exclusive for one campaign (chapter), but accessible for players of all campaigns.

For example:
 * The Battle Isles are the common high-level PvP area for all campaigns.
 * The core professions are the 6 professions that were already present in Chapter 1:
 * Monk
 * Warrior
 * Elementalist
 * Necromancer
 * Ranger
 * Mesmer
 * Each of the core professions also have a certain number of core skills.

For sorting purposes, it might be convinent to order core things as being chapter 0.