It occurs at the level of the subjective signs with which it is paired: The encoded material/subjective sign transforms when it is decoded because the subjective part changes.
One problem with Peirce's use of the term sign is it refers to two different things: material signs (Hall's sign-vehicle) and subjective signs (Peirce's interpretants).
It is this level of meaning that interests Hall: The potential for politics resides in the gap between producers' and viewers' subjective signs.
Material signs anchor different signification chains for producers and viewers: The material sign remains the same, but the subjective signs differ.
The program as a material sign is linked to different subjective signs when it is repeated.
In other words, context--the set of events that have shaped how we take a given word's prior uses into account--shapes the play between subjective signs.
My formulation shifts our attention to the relationship signaled by the arrows on the right and left of my diagram--the substitution of subjective signs from author to reader (or speaker to listener or producer to viewer) that takes place in every act of communication.
It is a specific type of communication (see Figure 2), one based on an exchange of material and subjective signs, both within a language (as people find new ways to express ideas the other does not understand) and between languages (as people search for ways in their new language to express ideas from their old one).