By definition, someone who self-identifies is basically someone who responds affirmatively to the question “Do you have a disability?” Defining disability is a thorny issue. The government has addressed it in a number of places throughout the federal policy infrastructure.
It would be tempting to think there is one definition that we could apply across the board. In fact, given that different pieces of policy have different intentions and are intended to address different sectors of the population, it's something that needs to be looked at very carefully.
Self-identification is a good start, but there needs to be a bit more thought than someone just coming forward and saying, “I have a disability and so I want to be considered for a particular program or a particular entitlement.”