I only really saw this point in the study design last week and was like "um what."
So it integrates bio, psych and social factors in that it takes into account that the stressor can be either biological, psychological or social and that the allostatic response depends on how the individual appraises that stressor (psychological factor). The brain then activates the physiological responses involved in the allostatic response (HPA axis, immune system, etc.) which is a biological factor. And then when the individual appraises the stressor as having been dealt with (psych factor), it takes into account social factors like emotional support, psych factors like if you're used a defence mechanism or biological factors and the brain turns off the allostatic response (biological factor)
I know it's messy, but I hope it helps a bit!