In a previous thread, an OP asked about mixed episodes vs. rapid cycling (ultradian cycling). I responded with things I had learned in DSMs and various books, but admittedly recognized that the topic brought up many questions in my mind, as well as confusion. I mentioned the term "mood lability" as one a psychologist friend mentioned to me when discussing significant mood issues that don't meet official episode criteria in manuals like the DSM-5. The topic of bipolar ultradian cycling vs. Borderline Personality Disorder was also hinted at in brief passing. It certainly is all very confusing these issues, and I know different psychiatrists have different views on them.

I decided to bring up the topics above to Dr. Jim Phelps, a well-known psychiatrist with vast experience, and a particular expert on bipolar spectrum disorders (particularly ones that don't always fit neatly in a category). He was kind enough to provide a link to an article he wrote on this topic, which also
contains links to other thoughts on these topics. He admittedly expresses disagreement with other experts, but comparing views can be interesting. I'm not suggesting his ideas are the absolute right ones, but he sure provides food for thought.

Here's his response to my email:
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Hello [my name] -
You have your finger on an area where our language is inadequate and often misleading [ultradian cycling vs. mixed states], as you've observed. So I think you'll appreciate an article in which this very issue is addressed, including the very blurry interface with "mood lability" and borderlinity. Here's my attempt to present the paper in plain English but you may want to get hold of the original.

Rapid Cycling And Mixed States As “Waves” | PsychEducation

I think this "waves" perspective helps a great deal in understanding rapid cycling (the very rapid kind) and mixed states. See if it makes sense for you -

Jim Phelps

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If interested, Dr. Phelps has written some highly regarded books, one of which I own. Though I have type 1 bipolar disorder, during a particularly long depression of mine my husband bought me his book Why Am I Still Depressed? Recognizing and Managing the Ups and Downs of Bipolar II and Soft Bipolar Disorder . It's a very interesting read for people with any type/form of bipolar disorder and even sheds some light on subtle differences between these main types. He also has a book about the bipolar spectrum. I have not yet read that book, but have read some of his views on this.