What's the difference between master and slave nodes in MySQL Cluster?
The data node marked as 'Master' in the output from the ndb_mgm show command is the coordinator of some cluster internal management tasks.  For example, it coordinates changes to the distributed dictionary due to DDL commands, and manages node join and leave transactions.  The 'Master' role is dynamic, and is generally assigned to the longest running data node in a cluster, only moving when that node fails.

The 'Master' role is completely unrelated to the 'Master/Slave' concept in MySQL replication, and also has no bearing on the replication of data between nodes within a cluster.  Perhaps it would be better if it were called 'Coordinator' or 'Leader'.  As you say, you do not need to worry about which data nodes are used by MySQLD, all are equivalent for most operations and it is handled automatically.