UCTD is a description of a condition that falls under the umbrella of connective tissue diseases but doesn't meet the sufficient criteria to be classified into a specific, well-defined CTD entity (SLE, scleroderma, RA). It is a relatively mild form of CTD.
This is different from mixed connective tissue disease, in which the symptoms and laboratory findings of the patient are sufficient for the diagnosis of more than one type of connective tissue disease.