The imp.cfg has comments on all the stats, so you can always look there for an explanation. This is what is says on those two:
Code:
; When a creature health falls below requirement, the creature will go to sleep.
; The value is scaled 0..255, where 0 means never go and 255 means go even if small
; fraction of health was lost. If a creature has Heal spell available, it will also
; try to use it when below requirement.
HealRequirement = 0
; When a creature health falls below threshold, computer player will pick the creature
; and place it in lair. This works for autonomous computer players and for assistance players.
; The value is scaled 0..255, where 0 means never pick and 255 means pick even if small
; fraction of health was lost.
HealThreshold = 0