I was curious about this, too, so looked in my DSM-5. I'm assuming your dad is from the US? The DSM-5 might not be completely the same as the diagnostic manual outside the US.

According to the DSM-5, I'm guessing (he should confirm this with his psychiatrist) that the "atypical" is based on a specifier for the bipolar type that he has (1, II, Cyclothymia, etc.). By specifier, I'm referring to extra descriptions like "with mixed features", "with rapid cycling", "with anxious distress", etc.

On page 151 of the DSM-5, the specifier "With atypical features" is described as follows:

"With atypical features: This specifier can be applied when these features predominate during the majority of days of the current or most recent major depressive episode.

A. Mood reactivity (i.e., mood brightens in response to actual or potential positive events).

B: Two (or more) of the following features:
1. Significant weight gain or increase in appetite
2. Hypersomnia
3. Leaden paralysis (i.e., heavy, leaden feeling in arms or legs).
4. A long-standing pattern of interpersonal rejection sensitivity (not limited to episodes of mood disturbance) that results in significant social or occupational impairment.

C. Criteria are not met for "with melancholic features" or "with catatonia" during the same episode.

Note: "Atypical depression" has historical significance..."

[I won't finish typing the note on this specifier. It's too much typing for me. They pretty much expand a little on the above.]

I think that if his doctor is not exactly referring to the above, that Tucson may have a point in the idea of a "lingo" or something similar. Given that, if I were your dad, I'd definitely ask why this "atypical" designation was given to him.