cache-dir-write
A deny rule: it refuses a write into this checkout's magus cache dir, which magus alone owns, and names what to run instead.
What it catches
A write into this checkout's magus cache dir, which magus alone owns.
Seeing it
A verdict names its rule in brackets, which is how you got here:
deny [cache-dir-write]: ...
magus describe rule cache-dir-write prints the same entry at a terminal, and
magus describe rules lists every rule this workspace enforces.