Psi and Talent abilities are the result of the combination of four specific Psi genes, and the degree to which each are active or interacting. Talents occur when a Psi gene is only half-active, and Psi abilities happen when genes are fully active.
Psi theory is really complicated, it involves genetics and the balance of different traits of each of the Psi Genes, as well as the unique skills and situations of each individual. So there’s no simple way of explaining. But I’ll try with different pages underneath this one on the pull down menu.
However, in general, the Agency system of rating Psi and Talent abilities is based around testability, i.e. repeatable, testable results. It revolves around concepts of range, strength, sensitivity, and control, which all combine to create a basic ranking system out of 5 (although with exceptional people like Jaola Armon, they’ve had to expand their rankings, but still out of five).
Talent is rated around a list of things that a particular skill can perform with increasing difficulty, and when you reach that difficulty it means you’ve reached that rank, which is usually out of ten. For example, Dodge Talent is rated where an assessor shoots at the person, their rating is based on successively bringing the shooter closer and closer to the person, where a level ten Dodge Talent is a person, with the gun barrel resting on their head, then being able to dodge out of the way of the bullet. Needless to say, some of the training procedures of the Agency are pretty brutal and sometimes fatal.