For example, if you're a Sorcerer or Wizard that chooses "project fire", you might get Firebolt, Burning Hands, Scorching Ray, Fireball, Wall of Fire, Firestorm, Meteor Swarm, etc. If you choose "project lightning", you get... Shocking Grasp, Lightning Bolt, and Chain Lightning. If you choose "project acid", you get Acid Splash and Melf's Acid Arrow and that's it.
Now you have to resolve the idea of if picking a track takes up a spell known, does each spell in the chain take a spell known? So Fireball takes 4 spells to know? That's obnoxious, especially for a Sorcerer.