Disability Services - documentation required?

[siredgar.livejournal.com]

I have two friends who will be coming this year who are likely to need disability services. One has hip problems, and the other will be post-surgery by a few weeks. Neither need wheel chairs or any overt assistance, but will have difficulty standing/walking for long periods. Last year while in line for a panel they wanted to sit in chairs that were available outside, but were told the chairs were reserved for those who had gone through disability services. This year they plan to do so, but particularly in light of not having obviously visible signs were wondering what documentation was required. Doctor's notes? Something else? Any input appreciated.