Try Googling the term (other search engines available).
From what I can see most GP Surgeries and Local Authorities maintain a register of people at risk of falling but not being on any such register most certainly does not bar a claimant from meeting the mobility criteria for PIP or ESA/UC.
However, don't forget that the distances used by these benefits are relatively short and there may be an assumption that someone would have to be at significant risk of falling over these distances for it to be a factor that would be considered.
Also, might someone who is at such risk opt for using a wheelchair or at least aids that reduced the actual risk of a fall such as a walking frame.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems