Short answer
Since partitions are inevitable, CAP forces a choice during a partition: a CP store rejects/blocks to avoid stale or divergent data (etcd, ZooKeeper, HBase) while an AP store stays available and serves possibly stale data (Cassandra, DynamoDB).
Explanation
PACELC adds the steady-state trade-off: Else, when there's no partition, you still choose Latency vs Consistency, which is why many AP systems offer tunable quorums. The decision is per-data-class, not per-system: account balances want CP, while a product catalog or feed tolerates AP/eventual consistency.
Common mistakes
- Saying you just pick two of C, A and P.
- Forgetting partitions are not optional, so the real choice is C or A during a partition.