SQL_CUSTOM is a Protocol were you can define SQL & Prepared Statements in a config file. This is more secure method & more powerfully options than using SQL Protocol.

The sql_custom config file is located @extDB3/sql_custom/foo.ini More info at SQL_CUSTOM INI FORMAT

9:ADD_DATABASE:<DATABASE_NAME>  
9:ADD_DATABASE_PROTOCOL:<DATABASE_NAME>:SQL_CUSTOM:<PROTOCOL_NAME>:<SQL_CUSTOM_FILENAME>  
0:<PROTOCOL_NAME>:existPlayerInfo:<player_uid>

<DATABASE_NAME> = Database Config Section in extdb-conf.ini  
<PROTOCOL_NAME> = Can be anything you want  
<SQL_CUSTOM_FILENAME> = @extDB3/sql_custom/<SQL_CUSTOM_FILENAME>

So for example to load SQL_CUSTOM, using database config [a3w] and a3wasteland.ini

9:ADD_DATABASE:a3w
9:ADD_DATABASE_PROTOCOL:a3w:SQL_CUSTOM:sql:a3wasteland.ini
0:sql:existPlayerInfo:123123123123

If extDB3 encounters unknown an issue / unknown option in sql_custom ini file. It will log the errors & not load the protocol. This should help trying to hunt down typo mistakes