A large voltage difference gradually creates a conductive leakage path across the surface of the material by forming a carbonized track.
Performance Level Categories (PLC) were introduced to avoid excessive implied precision and bias.
In terms of clearance, a higher CTI value means a lower minimum creepage distance required, and the closer two conductive parts can be.
The test method has an upper limit of 600VAC; higher voltages are currently not covered by the standard.
In the recent version of the standard the evaluation of the test method at higher voltages (above 600 V) is stated as a target for the future.