Drag a circle (while hovering over a reclaimable object) to reclaim metal or energy within the circle
Select a construction unit and drag a circle (while hovering over a reclaimable object) to reclaim everything (of the same type) within the circle.
Reclaim Area is a contextual Reclaim Command
So it will reclaim only what is directly underneath your cursor (metal or energy). Hold right mouse and drag an area to reclaim all features of that type in the area you select.
You can also Queue multiple circles with SHIFT. (or see command queuing)
TIP: If you want to keep reclaiming an area you can also turn on REPEAT.