Complex sentence: XP is gained via Story Quests, and as you gain more XP, it will unlock the ability to add more heroes to your roster, starting from the initial 2 up to 5 heroes and save more ISO-8, catalysts and objects in vault.
Split into simpler sentences: XP is gained via Story Quests and, as you gain more XP, players unlock the ability to add more heroes to their roster, starting from the initial two, up to five heroes. Higher levels also allow players to save more ISO-8, catalysts and objects in vault.