Those terms can have different meaning to different people.
To some people “situational depression” means depression that comes on after a stressful or traumatic event―for example, ending a relationship or losing a job. Other people use the term “situational depression” to refer to depression that is milder or shorter in duration.
The first definition of “situational depression” doesn’t tell us much about a recommended treatment. There is a traditional belief that depression following a stressful or traumatic event is more “psychological” than “biological,” in which case psychotherapy would be recommended over medication. But the evidence does not support that belief: both psychotherapy and medication are effective for the treatment of depression, whether it follows a specific event or not.
The second definition of “situational depression,” meaning milder or more transient depression, would be important in recommending treatment. Any specific treatment―medication or psychotherapy―is most clearly helpful when depression is more severe and longer-lasting. Depression that is mild or brief will often improve with time and good self-care. For milder or shorter-term depression, treatment with medication or psychotherapy is often not necessary; however, depression that is more severe and longer in duration is less likely to improve without specific treatment. We usually recommend psychotherapy and antidepressant medication for depression that is especially severe or long-lasting.
“Clinical depression” is not an official diagnosis, so it too can mean different things to different people. For some, “clinical depression” is another way of referring to the official diagnosis of major depression: at least moderately severe and lasting longer than two weeks. As described above, depression that is more severe and longer-lasting is less likely to get better without specific treatment. If “clinical depression” means “major depression,” then it is the kind of depression for which antidepressant medications and/or psychotherapy are likely to be helpful.