Version: Odoo 11

In the openacademy tutorial, there is this block code in the xml file:

       <act_window id="launch_session_wizard"
                    name="Add Attendees"
                    src_model="openacademy.session"
                    res_model="openacademy.wizard"
                    view_mode="form"
                    target="new"
                    key2="client_action_multi"/>

Where can i find more information about key2 parameter, its possible Action Event values and what do they do ?