To specify the authentication plugin names, an application must create a config string with "AuthClient = <plugin-list>" (+ lines for other options). However there exists a specific DPB/SPB item isc_dpb_auth_plugin_list/isc_spb_auth_plugin_list that the client itself uses to communicate the plugin list to the server. However if an application populates that same item, it is ignored by the client.
Allow an application to use isc_dpb_auth_plugin_list/isc_spb_auth_plugin_list instead of having to create a config string.
See also discussion on firebird-devel titled "Passing plugin list to native client".