Depression. Melancholia can be a feature of severe depression. Not all severe depressive disorders have this as the main symptom. Although there is a relationship with severity.
There can be. Major depressive disorder is diagnosed by having at least 5 symptoms from a list. Melancholic major depressive disorder refers to having certain specific symptoms. The severe classifier refers to how much the symptoms you have interfere with your functioning and your life. Much of the time if you have melanchotic mdd your symptoms are severe but not always.
A subtype. Melancholia is a subtype of depresson which generally is more severe with lack of reactivity in mood. There are more physical symptoms of no drive, motivation, apathy but can have a dramatic change from am to pm with much worse mood in the am. Melancolic depression is less responsive to therapy alone and often requires multiple medications or ect.