There are no 'rules' on this. I would mention it whenever and wherever it seems appropriate. This would vary for one person to another. For instance someone may have difficulty getting dressed because of severe arthritis and say that it takes them such a long time that they have to start the process very early in the morning in order to not be late for work. A person might mention work under the "communicating with others" or the mobility questions. Someone could choose to put the information on an additional sheet.
If your medical condition impacts your work, make sure you say so. If your employer has made any reasonable adjustments, you should mention them. Be aware that assessors will often jump in and make assumptions if you don't make any limitations you have at work clear.
BIS
Nothing on this board constitutes legal advice - always consult a professional about specific problems
