RED — for services
Rate: requests per second. Errors: % of failed requests. Duration: latency distribution (p50, p95, p99). One service = three core metrics. If you can't see all three, you can't tell if it's healthy.
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USE — for resources
Utilization: % of resource in use (CPU, mem, disk). Saturation: queue depth, waiting work (CPU run queue, I/O wait). Errors: hardware errors, retries. Resource = one node, disk, network interface. Catches bottlenecks before they cause service-level issues.
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Why both
RED tells you 'service is slow'. USE tells you 'why' — CPU saturated, disk full, network errored. RED is user-facing, USE is operator-facing. SREs need both; engineers usually only see RED.