Is MySQL Cluster just for in-memory databases?

In MySQL website I see below definition for MySQL Cluster. > MySQL Cluster is a technology that enables clustering of in-memory > databases in a shared-nothing system. I want to use Clustering for a chat application like viber that there are some Ejabberd servers, which use MySQL as database.Is it necessary that databases use in-memory tables. according this definition all databases and tables should be in-memory or not? This is the link of MySQL Cluster definition.