I don’t know what OP might be thinking of, but I can give you an example. DSM-5 does not recognize Complex Post Traumatic Stress Disorder (C-PTSD), which is associated with chronic traumatizing experiences (e.g., victims of physically abusive parents, victims of sex trafficking). The diagnostic criteria would be different than the currently recognized PTSD, which tends to be based on one or a few traumatic events (eg., soldiers/survivors of war, car crash, rape). Since it’s not recognized, many people who have PTSD-like symptoms but who don’t fit the current criteria get diagnosed with anxiety or other disorders, and subsequently don’t get access to the most effective treatments. CPTSD exists on a spectrum from sub-clinical to disabling, just like PTSD. People on the extreme end should get the necessary accommodations for their disability, but without the clinical diagnosis are often expected to kind of suck it up in the “everyone has anxiety” kind of way.
Bessel van der Kolk and other mental health experts/clinicians have been working for decades to get it included in the DSM, but it continues to be excluded. It is, however, in the ICD-11.