https://docs.atlas.mongodb.com/reference/api/clusters/

The Atlas API documentation should be updated to reflect the mongoDBMajorVersion field being included in responses.

Calls to the clusters endpoint return a response that contains both the mongoDBVersion and mongoDBMajorVersion fields. However, only the mongoDBVersion is depicted in the documentation. The mongoDBVersion field is read-only, however, it should also be noted that Atlas clusters can be upgraded from version 3.2 to 3.4 by making the Modify a Cluster API call with the mongoDBMajorVersion field changed from "3.2" to "3.4" accordingly.