Actions are possible on multiple items starting with Magnolia 5.2 MGNLUI-410. This allows the user to for example delete all selected contacts.

• Document action bar group availability in Action bar definition. Add a new section Availability, like is done in Action definition
• Explain what availability means in the action bar context. This is already explained in Action definition, "Action availability is different from action group availability. Group availability defines whether the group of actions is displayed in the action bar. If the group is displayed, then action availability is checked for each action in the group."
• Document multiselection availability. The Contacts app has an example in /modules/contacts/apps/contacts/subApps/browser/actionbar/sections/multiple. You basically list the groups and actions where acting on multiple items is permitted.