Applications such as SAP systems use interfaces to access SAP MaxDB databases.
If the application (and thus the SAP MaxDB interface) is on a different computer than the database, the database system also needs the SAP MaxDB X Server (communication server) for communication. Note that the JDBC interface needs the X Server even if it is installed on the same computer as the database.
More information: X Server documentation
Database Applications Accessing an SAP MaxDB Database via SAP MaxDB Interfaces
Access to a Document Store in an SAP MaxDB Database via the SAP MaxDB WebDAV Servlet