The most common target of reclamation is the wreckages that former units leave behind when they die. Reclaiming a unit's wreckage will give back 81% of the mass used to construct it, unless the wreckage is further damaged by attacks or area effect damage. In addition to units, certain ambient objects such as rocks and trees can be reclaimed as well. While their value is individually quite low this is a free way to boost the economy without having to venture to faraway mass deposits and can be a safe way to expand the economy without taking more area, especially at T1 before the advent of mass fabricators. Rocks primarily provide mass and trees primarily provide energy. This can allow the early production of power generators, enable early upgrades to mass extractors or act as a delaying replacement to early power entirely. Overall, without reclaim, especially the wreckages of t3 and experimental units, the player will be at a severe disadvantage.
Reclaim is automatically included in the patrol and attack move commands for engineers when the mass storage is not full. While engineers and factories cannot be issued the attack move command from the panel, alt+rightclick acts the same as an attack move command. However, it is faster to manually position the engineer and queue manual reclaim.
Live units can also be reclaimed, regardless of whose faction they belong to. Reclaiming a live unit will not give back any mass so if the goal is to recycle your own units, press ctrl+k to kill them first. This is primarily useful for engineers to defend themselves against enemy engineers or T1 land scouts. The only exception to reclaim targeting is Armored Command Units, which can never be reclaimed.
The only units which can reclaim are Engineers, Armored Command Units, Support Armored Command Units, Engineering drones, the Cybran Experimental Megabot, and the Aeon T3 Heavy Assault Bot. While some other units have engineering suites such as the Hive Engineering Station, these cannot reclaim.
You can efficiently cut a way through walls by ordering any unit capable of reclaiming to go on a patrol that will lead it near or through the wall. This is because placing any unit capable of reclaiming on patrol will cause it to automatically reclaim any trees, rocks or wreckage within range. However, be warned that if you want to use this technique to remove enemy walls, combat units which are able to reclaim will prioritize fighting over reclaiming when enemy units are nearby. You'll have to clear away any enemy units near the wall before a patrolling combat unit will reclaim a hole in it. Reclaiming is also helpful in delaying the complete exhaustion of your mass supply (Mass Stall) if current expenditure exceeds production.