First a wreckage is a
prop, this means that it behaves just like a rock, except for the posibility to build the same thing on top of it for 50%price in the case of building wreckages. Not really important, but just something that irked me.
Each prop has a certain amount of max hp (some % of original unit hp in the case of wreckages) And whenever a wreckage gets damaged for x% of its hp then it also loses x% of its reclaimable mass and energy. Wreckages will also contain less mass than is necessary to build it in the first place.
ReclaimingYou can't actually reclaim a 'live' unit. Doing a reclaim command on a 'live' unit will slowly remove its hp to 0, at which point the unit dies and becomes a wreckage. During this process you gain no resources. The dead unit now left behind a wreckage and from then on you actually start properly reclaiming and you gain mass. So you don't win anything by reclaiming a 'live' unit compared to first ctrl-king it, but sometimes ctrl-king can have nasty side-effects.
So if you can ctr-k something then that will remove the time needed to 'kill' the unit. So you basicly saved some time and got the mass faster.
Unfortunately wreckages can get damaged and with the hp loss comes mass loss. So if you have a volatile building next to reclaim or next to another unit that you want to reclaim at the same time, then ctr-king all of them is dangerous. Because the death damage will hit the reclaim (even wreckages of units that got ctrl-ked at the same time) and lower its value as explained before.
For example: ctrl-king a line of pgens to reclaim them, will give you less mass than reclaiming the 'live' pgens. (no death damage when it dies due to getting reclaimed)
So ideally you ctrl-k things to speed up the reclaim, while avoiding to damage any of the wrecks. It depends on the situation and what you want: less mass faster or more mass later.
RepairingUsually one doesn't repair units, because it costs the same amount to build it to repair it. For example repairing 50% of a pgen will cost 50% of the mass and energy to build a pgen.
Since damaged units behave exactly the same there usually is no reason to repair them, because getting a second full hp one is usually better than having only 1 full hp one. So unless the unit is under threat you won't notice any difference from a 1hp pgen and a full hp pgen.
Ofcourse this also is situationally depend because if keeping that 1 unit alive will win/save the game then getting a 2nd one maybe isnt a good choice.
Repair vs rebuild? (if the unit really needs to be full hp for some reason)
If the unit is providing an important feature that you can't go without during the time it would take to rebuild it (for example a t3 pgen or defending experimantal unit) then ctrl-k and rebuild isn't an option. But if you can spare the feature for the rebuild time duration then ctrl-k can be a viable option.
Incase of mobile units you can simply reclaim it and get it for ~70% less mass, but you still pay 100% of the energy and build time. So this is usefull for getting your experimantals back to full hp for a lot less mass than repairing an almost dead one would.
Incase of buildings you have 2 options: reclaim the mass or build same thing on top of it to get it already build at 50% (i think it decreases whenever the wreck is damaged so 1hp wreck won't give much start %)
Why would you reclaim the mass?You get more than 50% of the mass for reclaiming it compared to having to start at 50%. So if only mass is the limiting factor it is better to just reclaim it than to rebuild it. Beware of this when you rebuild t2-3 mexes and other important high mass buildings.
Why would you rebuild on top of the wreck?When you are also limited in energy or buildpower, it could be better to just start at 50%. So if you getting that building back up is timesensitive then being able to shave off 50% of the buildtime can help you a lot. Could be useful when rebuilding t3 artillery or paragons.
Or when you lose that all important first t2-3pgen then you'll be in need of energy so getting a 50% energy discount will be very useful for getting it back up in reasonable time.
Fun facts about unfinished buildingsWhen reclaiming an unfinished building you do get
all the mass and energy you put in it back. So if you reconsider your building plans it is good decision to reclaim the unfinished building instead of letting all the juicy mass and energy go to waste. Don't wait too long though because an unfinished building will decay when not being worked on so you're losing mass each tick.
This knowledge can be used in the tight situation where all your t2 or t3 pgens get sniped and now you are in total power stall. Just rebuild a pgen on top of each wreck and turn these 2 50% unfinished buildings into 1 finished building by reclaiming 1 of them so you can put the energy you gain from it into the other one.
Hope this helped you.
PS: f***, this took way too long... I only wanted to write 1-2 paragraphs...
FAF wasting my time since 2015