When is damage "permanent" in Accel World? Within the accelerated world, when someone gets damaged or injured (even severely), they fully recover after leaving and re-entering the accelerated realm. (If you are killed you also come back normally.)

For example:


Taku cuts off Haruyuki's arm. (ep 5)
Niko's left arm is ripped off when Yellow Radio's minions tear apart her fortress. (ep 11)




Both of these injuries (a loss of an entire limb) are serious injuries that can't be recovered in battle.* However, when they leave and re-enter the accelerated world, they are fully recovered.

*Barring Chiyuri's time-reversal ability.



But there are at least two instances of when damage is not reversed upon re-entering the accelerated world:


Kuroyukihime amputates Fuuko's legs - hence the wheelchair. (described in ep 16)
Noumi steals Haruyuki's wings. (ep 15)




So what determines whether damage is healed?

Furthermore, how does Chiyuri's time-reversal ability play into this?
It seems not to be some kind of "rule", but aspect of specific stronger abilities/avatars:


it was explicitly said that Black Lotus avatar inflicts permanent damage with her blades
"steal" ability wouldn't be much of a steal without permanent aspect to it


I don't think it was explored (in anime at least) if there are ways to purposely achieve it (from leveling up would be logical) or as other aspects of avatars some people just happen to have it.