Line of Work.
Language as process: the moving of meaning across the gap between one mind and another, and the persona's use of language with itself underneath that. The domain does not model grammar, vocabulary, or proficiency; it models the loss that opens at every step of the loop.
The spine is the communication ladder popularly attributed to Konrad Lorenz: thought is not said, said is not heard, heard is not understood, understood is not agreed. Each arrow is a place the meaning can crack. The engine takes that chain literally and gives each channel its own loop, so the loss is recorded where it happens (what was heard is not what was understood, what was understood is not what was said), and widens for a persona who must cross it in a language that is not yet the body.