I'm trying to use afterSubTpl but I have no idea how to use it. This what I have tried so far.

Code:
{
    xtype: "combo",
    displayField: 'department_abvr',
    valueField: 'department_abvr',
    queryMode: 'local',
    typeAhead: true,
    forceSelection: true,
    fieldLabel: 'Department',
    name: 'department',
    store: departmentStore,
    allowBlank: false,
    afterSubTpl: '{department_name}'
}
What I'm missing here?