In Castaneda's works, the term means a locus of perception within the energy field of a being.
Moving the assemblage point causes the being to perceive and interact with a different reality. This could be likened to tuning to a specific channel of reality. Castaneda describes the universe at large as consisting of filaments of energy. A being is an egg-shaped field of energy and some filaments pass through it and others do not. The filaments which intersect with the being are the sum total of realities accessible to the being.
All shape changing, travel between worlds and states of consciousness, etc. discussed by Castaneda derive from shifting the assemblage point. This can occur in a somewhat haphazard manner through the use of drugs but ultimately is something to be accomplished as an act of will.
The concept is specific to Castaneda. A somewhat corresponding concept in the Cassiopaea material is the notion of frequency resonance envelope or realm. Essentially all these concepts attempt to bring into words the relationship between consciousness and its external reality and the laws governing this reality.