This selects whether the device chooses which TACACS server to contact strictly in configured priority order, or uses a cached result, presumably remembering the last server that successfully responded, to skip straight back to a known-good server. The cached method trades a small chance of missing a genuinely-higher-priority server having since come back for faster, more predictable login response times by skipping the failed-server probing sequence on every request. For example, after a primary TACACS server outage causes the device to successfully authenticate against a backup server, setting this to cached(2) would make the device keep using that backup directly on subsequent logins rather than re-trying the still-down primary every time.