For example, a circle will have one handle to control the position of its center, and another to control its radius.
Handles are displayed in the Work Area for all the currently selected layers and for all the currently selected ValueNodes in the Children Panel. If you are seeing handles you don't expect to see, check in the Children Panel - you probably have ValueNodes selected in there.
Move the Handle
Drag the handles around to change the parameters. Hold while dragging to constrain the movement to be either horizontal or vertical.
More precise control of the handles can be achieved using the cursor keys. The cursor keys will move the currently selected handle one pixel at a time, or ten pixels at a time if the key is pressed.
Choose your handles
More than one handle can be selected at a time. Hold while clicking on a handle to toggle the selection on and off without affecting the selection status of the other handles. Drag a rectangle around a group of handles to select them all and unselect the other handles. Hold while dragging a rectangle around a group of handles to select them as well as the currently selected handles. Hit to select all non-position handles.