The thread is processing events for binary logging.
The thread is doing the work of schema replication.
This is used to have a correct binary log of schema operations for NDB.
The thread is waiting for permission to take a global schema lock.
The server is acting as an SQL node in a MySQL Cluster, and is connected to a cluster management node.
The thread is waiting for a global schema lock held by another thread to be released.
The thread is waiting for a schema epoch (that is, a global checkpoint).