What is it about the nature of disability that you want to assimilate?
The nature of disability is a complex animal to define in and of itself. The reasons, background and origin of BIID is just as complex, and mis-understood. This is not a simple, nor straightforward question to answer.
For most of us, it's not about having a disability, and therefore it is impossible to answer about the nature of disability. It is a question of body image. It is a question of needing to be (emotionally) comfortable in our own skin. The fact that an amputation, or paralysis, or deafness, or blindness are considered disabilities is actually irrelevant to the equation of BIID.
Most of the people in this group are already disabled, in that the desire to be an amputee (or whatever) can become so intense, debilitating and depressing, that nothing else matters. It can reach a stage where the desire to be rid of the limb can focus the mind to the exclusion of every other aspect of life. This is a critical time, as the sufferer may resort to dangerous and life-threatening self-harm to resolve their despair. While the focus is on the desired transformation, the sufferer is usually resigned to the possibility of dying in the attempt, and proceeding regardless.
That said, for some people, there is an aspect of BIID that relates to wanting to overcome the challenges related to the required impairment. For other people, there is an aspect of wanting to belong to a social group.