Generally, these are expenses that can be paid with tax-free dollars through an account like an HRA, FSA, or HSA.
These expenses are not listed but rather are described in Internal Revenue Code section 213(d): https://www.law.cornell.edu/uscode/text/26/213
Most TPAs that administer tax-advantaged accounts offer a listing of eligible expenses.
These sorts of expenses are also described in IRS publication 502: https://www.irs.gov/pub/irs-pdf/p502.pdf