Component selection modes
These buttons let you switch between component selection modes:
Multi-component mode
When multi-component mode is active, all of the component selection mode buttons are highlighted in the Modeling Toolkit window: 
Multi-component mode lets you work with all three component types without changing between selection modes. When entering multi-component mode from a single selection mode, your selection is automatically saved.
To enter multi-component mode, click the Multi-Component button, select Select > Multi-Component from Maya's main menu, or use the hotkey 
+ Tab.
Convert a selection
-click a selection mode icon to convert your current selection to that component type.
For example, to convert vertices to faces, select the vertices in your scene, and then -click the face selection mode icon. 
-click the face selection mode icon.