Every bound needs a predicate: a short, fixed vocabulary word that says exactly how its subject knot relates to its object knot. Kastalia constrains this vocabulary deliberately -- predicates live in a proper Postgres ENUM type, not a free-text column -- so that a graph query can rely on a closed, known set of relation kinds rather than accidentally-inconsistent free text.
This short mention is just a pointer: the full catalogue of predicates, with one knot describing each, lives in its own section of this guide (see
Predicates, further down).