Server's Variable: default_tmp_storage_engine - Documentation Unclear

MariaDB "default_tmp_storage_engine" System's Variable is not clear enough.

• By default it has a value of NULL. ¿What does that mean? ¿That the default server's engine for temporary tables (CREATE TEMPORARY TABLE) is that from "default_storage_engine"? Documentation is not clear enough.

• Also, when choosing "Memory" Engine for tables created via "CREATE TEMPORARY TABLE", when exceeded "max_heap_table_size", ¿Are tables automatically converted to Aria Engine? (As it happens with internal temporary tables)

  SET @@default_tmp_storage_engine = memory;

Thank you,
Juan