Several of Thermal Expansion's blocks and items follow a set tier system. This tier system consists of four tiers: Basic/Leadstone, Hardened, Reinforced/Redstone and Resonant. This page describes each of those tiers.
Some items from Thermal Dynamics also follow this tier system. However, a fifth tier called Signalum is also present there: an intermediary tier between Reinforced/Redstone and Resonant.
The following blocks and items currently follow this tier system:
Common Materials: Invar, Redstone (solid), Glass
This tier is a slight improvement over the first tier; usually obtained by adding some Invar to items of that tier.
This tier replaces solid Redstone with the much more conductive Destabilized Redstone. As a result, this tier is a rather large jump from the earlier tiers.
Only present for certain items from Thermal Dynamics. Items of this tier are between the Reinforced/Redstone and Resonant tiers in terms of how powerful they are.
Currently the most powerful tier. A significant improvement over the Reinforced/Redstone tier, usually obtained by adding some Enderium to items of that tier.
Common Materials: None
Devices of this tier cannot be legitimately obtained. They are usually capable of things that would be unacceptable in normal play, but can be useful for testing. Usually this involves providing an infinite amount of something, or being able to do something an unlimited amount of times. Creative-tier devices are usually colored magenta, and have a big C on them.