The Engineer is the standard building/repair unit common to each faction. Engineers carry no form of weapons (excluding Cybran 'Direct-Fire Turret' upgrade), so they are virtually helpless if unescorted by combat units.
Engineers have the following abilities:
Build. Like the ACU, an Engineer can build structures, although it cannot construct these buildings as fast as the ACU.
Reclaim. Reclaiming the risidual particels of working and/or destroyed units will give you a boost to Mass and Energy.It can also be used as a weapon by reclaiming enemy mass from units or structures.
Repair. Engineers will automatically repair units or structures in the vicinity, at no charge. You can also directly order your engineer to repair a unit or structure.
Capture. An engineer can capture an enemy unit or structure, converting it to your side. This can be tricky if you are attempting to capture another enemy combat unit. Capturing an engineer or factory of a different faction allows you to build units and structures that your faction cannot usually produce, you can use research points to upgrade captured units or unlock advanced units from other factions. when you build and or capture a research station that is from a different faction you go on the research tree and right near your factions symbol is 2 arrows, when you press one of those arrows it shows you the research tree for the other faction from there you can research anything from that faction and build their units (with their engineer). The Illuminate Engineer can sacrifice itself to quickly capture an enemy.
Patrol. Engineers sent on patrol will, whenever possible, reclaim Mass, capture units, and repair units found in their patrol route.
Engineers will automatically repair units, at no additional cost. This makes it important to send engineers with each attacking force to repair units that have been damaged.
Engineers will also automatically reclaim wreckage near them, meaning that you can just send them to a wrecakge-rich area to get more mass.
The Cybran engineer can be upgraded with a direct-fire weapon. Combined with the +100% repair rate they are a worthy foe for land units.
It may be beneficial to place one or two engineers behind a collection of point or air defenses. They can keep them in repair while also collecting Mass from wreckages.
Submerged units may be captured/reclaimed (thus forcing a lone unit to surface or run away).
Sacrifice Capture will capture at an accelerated speed in exchange for destroying the engineer.
During early stages of the game, if you can research your factions transport unit and quickly ferry off 10+ engineers to an opponents base, you can devastate your opponents production by capturing his/her Mass Extractors and Energy Generators, or annoy your opponent by building an excessive amount of Point Defense Towers right at his position, which could potentialy score you an early win.
Using the same strategy above, you can use it in the late game, capture your opponents set-alone Mass Extractor farms or light-weight forward bases setting up your attacks.
in late game, or games with pre-research active (exclusion: no research, all research and units unlocked) a UEF player can utilize the NOAH unit cannon and the engineer's radar/vision to make them effective scouts.
when destroying an enemy base throw a few standard engineers into the queue of one or two of your factories so when the base is destroyed you can instantly claim the space for your own use (this can be important on maps with low build space, like geothermal borehole as the land will be quickly fought over).
Note: there is an umpatched glitch in xbox versions that allow you to use the 'farming' technique. As most online players do this glitch, it should only be fair to new players to do the same( I am not encouraging any player to do this, I would reccomend it for single player, as you will not hurt anybody else) To preform this task is simple, take two engineers and assign one of them to build any building, (anti-air is preferred) While the builder starts this take the other builder and assign it to repair, the now constructing building. After you have done that, instantly assign the builder to then deconstruct it, giving you a mass and energy boost, (all of this must be completed before the tower is built, otherwise it will deconstruct and the boost will wear off(P.S. this takes up 3 units, but is better than 4 mass production units, and 6 energy factories, however it will be air defenseless, and therefore easily killable by all, so protect it good.)