My advice to people with ME/CFIDS is to use depression as the reason for disabilty. It is something they understand and depression (cognitive dysfunction and fatigue) symptoms are encompassed by ME/CFIDS. Also be aware that for political reasons, they rather approve you for that than ME/CFIDS.
This is bad advice because it constitutes fraud if you aren't depressed.
Often people with chronic illnesses have accompanying depression which further impedes ability to function.
There are no specific mental health symptoms eg cognitive dysfunction that guarantee disability approval. What is the focus of disability assessments is functional capacity to work rather than specific impairments or diagnoses.
No one is disabled simply because of a diagnosis. It is the severity of your symptoms and limitations and how they impact on your life. For example, in high school I had a friend who was in a wheelchair due to a birth defect, she could walk short distances with the aid of crutches. She is gainfully employed at radio station full-time. Then there are people with ME who have such severe symptoms and physical limitations that the use of a wheelchair is necessary despite the fact that there is nothing physically wrong with their legs.
They will likely ask you questions about your activities of daily living re: what are you physically still able to do in the home, at work etc. Success of getting disability = the inability to work at any job for any length of time. If a doctor writes that you can work part-time, that will not get you disability, they have to state you can't work at all and that your illness is not going to resolve any time soon.
You should probably prepare yourself by going through your worst day and thinking about your limitations, eg:
How long can you sit up for?
How long can you stand for?
When do you 'crash'?
Are you able to read, comprehend what you are reading?
Does pain impair your functioning?
How much sleep do you get, is it unrefreshing, does it contribute to cognitive function?
What household chores can you do if any?
Are you able to bathe/shower independently?
Are you able to shop?
Do you have difficulty talking on the phone, carrying on conversations?
What kind of help are you presently receiving for things you used to be able to do yourself?
Can you still drive?
How do you cope in a noisy environment?
Hopefully, you have medical records that will support your claims.