Use when working with MongoDB — designing a schema or collection (embed vs reference, data modeling, BSON types, schema validation), optimizing performance (indexes, ESR rule, explain() / slow query tuning, aggregation pipelines, covered queries), benchmarking or monitoring (profiler, mongostat, working set, executionStats), or applying standard practices (write/read concern, read preference, transactions, replication, sharding/shard keys, security, connection pooling). Covers MongoDB 8.0 with version-gated notes back to 4.x.