When the handle Action is set to Command, this block appears at the bottom of the interface.  Use the pulldown to select from one of the many commands that you can set to happen when you click the handle icon.

For example, in the image above, when you click on the icon it will open up the coordinates chooser on the bottom right of the interface.  Keep in mind that each handle can have three different mouse-click actions:  left mouse button, middle mouse button, and right mouse button.

When the handle Action is set to Run Expression or Compute Expression, this block appears at the bottom of the interface.  In the Exp field you can enter the expression that you want to execute when the handle icon is clicked or dragged (depending upon the action you have chosen on the Handle block).

For example, in the image above, clicking on the handle would set the Item List visibility to All Items.  (See Tips & Tricks.)

First choose what type of action you want to happen, Visible, Hidden, or Toggle.  This is so a handle can control the visibility of other handles.

Then choose what should be affected when you click.  The choices are:  Self (just affect this handle), Siblings (affect this object and others that are in the same place in the hierarchy), and Children (only affect the handles that are children of this one.

Most common setting: Toggle--Children (or Siblings)

This is for when you are using the Target drag type.  Set the Controller to be the item that will look at the target, which you choose here.

For example, if you set the Controller to be the Camera, then whatever you choose in this list will be what the camera looks at.  This will override the camera's rotation controls.

Double-click on the black arrow icon at the top left of the World View to see a sample.  Then double-click again to make those buttons reappear.