The CE is such a nifty way to summarise the feel of the story! Fantabulous.
And, I also have a working theory that the MC CE is the element that ushers the MC into the OS. I haven’t tested it out on a whole lot of storyforms yet so don’t hold me to it.
For example, in Nightcrawler, our MC is scrounging around for a job and some cash. That’s his Situation/Present.
OS is completely different in that it’s all about capturing and delivering (and twisting) the events for the news.
Two different line of events up until the CE kicks in. Plus, when writing a story, a writer is always looking for a way to braid the MC into the OS in a credible way.
In Nightcrawler, the MC CE is Potential. What sparks his connection to the OS is the Potential element...in this case, Lou Bloom sees the potential of what he could do to make money...and off the story goes.
Just a working theory....