Explanation

HSQLDB is a relational database written in Java. XWiki can use it as an embedded or remote database.

This database operates in two modes:

• Embedded: The database runs inside the same JVM as xWiki. No separate server process is needed. Configuration is simple because the database requires no installation or startup before use. 
• Remote server: A standalone HSQLDB server process runs independetely. XWiki connects to it over the network.

For production environments, consider the following limitations:

• Memory usage: HSQLDB stores the full database in memory. A wiki with 9 subwikis default XWiki pages requires aproximately 110MB of memory.
• Persistance: Data is written to disk periodically, but in-memory storage means that insufficient heap size firectly impact stability.
• Operational tooling: HSQLDB lacks built-in backup, replication and amdinistration tools comparable to production-oriented databases.

For prodduction deployments, consider using MySQL, MariaDB, or PostgreSQL.