As the spell's name states this is atomic regeneration, this spell will heal anything from scrapes and bruises to lost limbs, however this spell does not return the dead to the land of the living.
For healing effects of this spell you will roll a 4d8 to roll for hit points. You may choose up to 8 target creatures of your choice or subsequently 8 targets. Meaning you have 8 instances to roll for. This means you may choose to heal a single creature for all 8 instances or divide all instances how you see fit.
For healing lost limbs this is dependent on your concentration you must roll a d20 anything higher than your spell dc is a success any roll lower than that will result in a failed result and you will have to try again.
At higher levels the healing will increase by 2d8 for each higher level the spell is cast at, and for healing lost limbs and appendages, for each higher level you cast you may decrease your spell dc to increase the chances of a successful result.