In the Cassiopaea material, this refers to an archetypal idea. The idea is related to the idea of a Platonic ideal form of something, existing outside of space and time.
The ray of creation starts with a division in two distinct thought centers, these being of 'being' and 'non-being', also known as service to others/radiance/expansion and service to self/absorption/contraction. The term used will depend on the context and on what aspect of this fundamental duality one is discussing.
These thought centers then divide and branch into all conceivable variations of these themes. A thought center is not limited to any particular density. They can be thought of as orthogonal to density, with the essential idea of the thought center finding a density specific manifestation at each level.
For example, a subdivision of the thought center of non-being can manifest in 2nd density as the crocodile species, in 3rd as a certain type of psychopath and in 4th density as the 'lizzie.' The invariant idea here is a form of predation.
An individual can be seen as a crossroads of thought centers. The individual's free will consists in selecting alignment to one thought center or another. Thought centers interact with creation by being expressed by beings.
The concept is closely related to the Sufi idea of 'name of God.' The Ra material also discusses the theme under the name 'distorsion of the Law of One.'