Cosmos DB access models: NoSQL (native API, JSON documents with SQL-like queries), MongoDB, Apache Cassandra (column families, CQL), Apache Gremlin (graphs), Table (key-value), and PostgreSQL (distributed relational).
In DP-900: new app with JSON → NoSQL; migrating apps that use MongoDB drivers → API for MongoDB; traversing relationships → Gremlin. Cosmos DB for PostgreSQL is not recommended for new projects, and the MongoDB vCore option is now called Azure DocumentDB.
Azure Cosmos DB APIs is a concept that can show up on the exam. Check the full guide to see it in context.