If you are claiming your fiancé as your dependent under the Qualifying Relative rules her being disabled has no affect on your tax return or receiving an additional credit.
To be a Qualifying Relative -
1. The person cannot be your qualifying child or the qualifying child of any other taxpayer. A child is not the qualifying child of any other taxpayer if the child's parent (or any other person for whom the child is defined as a qualifying child) is not required to file an income tax return or files an income tax return only to get a refund on income tax withheld.
2. The person either (a) must be related to you or (b) must live with you all year as a member of your household.
3. The person's gross income for the year must be less than $4,300 (social security does not count) in 2020
4. You must provide more than half of the person's total support for the year.
5. The person must be a U.S. citizen or a U.S., Canada, or Mexico resident for some part of the year.
6. The person must not file a joint return with their spouse.
You are not disabled.
If you are living unmarried with your partner, you are single, and file a tax return using single status. (Or, if you provide care in your home for a qualifying child dependent, you might be able to file as Head of Household.)
Either way, you list your income and deductions. "You" in the program refers to you, not your partner.
Your partner might qualify to be claimed as a tax dependent, and if so, you can check the box in the dependent information section that your dependent is disabled. But "you" are not dependent.
Separately, the tax definition of "disabled" is "unable to perform gainful work due to a condition that is permanent, or will last at least one year, or will lead to death." Many people who meet the definition of disabled for other purpose, but who can still perform "gainful work" are not disabled for income tax purposes. For example, many "disabled veterans" can have service-connected disabilities like amputations, and qualify for VA benefits, but can still work at normal jobs (with accommodations) and so are not disabled for tax purposes.
For tax purposes, disability is only relevant for children. Even then, a child's disability doesn't affect tax filing until he turns 19 (24 if a full time student). Then his disability allows him to continue to be considered a "Qualifying Child" for tax purposes regardless of age.
But to answer your question, No specific medical diagnosis constitutes disabled to tax purposes.
“The term ‘disability’ means, with respect to an individual –
(a) a physical or mental impairment that substantially limits one or more of the major life activities of such individual;
(b) a record of such impairment; or
(c) being regarded as having such an impairment.”
For the IRS, disabled means -
"an individual shall be considered to be disabled if he is unable to engage in any substantial gainful activity by reason of any medically determinable physical or mental impairment which can be expected to result in death or to be of long-continued and indefinite duration. An individual shall not be considered to be disabled unless he furnishes proof of the existence thereof in such form and manner as the Secretary may require. "
I believe the substance of that paragraph is that it is your Doctor's decision