Display a series of buttons together in a single line button group. It is a "radio" field, only one button can be selected at once.
array(
'id' => 'button_bar_1',
'type' => 'button_bar',
'title' => 'Button bar',
'options' => array(
'one' => 'One',
'two' => 'Two',
'three' => 'Three',
),
'default' => 'two',
),