Allow clients to inform servers during isMaster which user they will eventually authenticate as. This permits the server to express which SASL authentication mechanisms may be used to authenticate as that user. The username send in the command request must be unprocessed and unnormalized.

Example:

MongoDB Enterprise > db.runCommand({isMaster: 1, saslSupportedMechs: "sajack.admin"})
{
        "ismaster" : true,
        "saslSupportedMechs": ["SCRAM-SHA-1", "SCRAM-SHA-256"],
        ...
        "ok" : 1
}