People use "upper middle" to mean different things. Some use it to mean the upper portion of the middle class, by income. Others use it to mean a distinct (and much smaller) class between the middle and upper, with distinct tendencies and preferences, comprising people who live something close to a "rich" lifestyle but are distinguished from the actually-rich by having to work to maintain it, rather than being able to live a very nice life off investment income alone. Doctors, lawyers, other very-well-paid white-collar types, constituting perhaps a single-digit percentage of the population. I'm not sure which, if any, definition has a credible claim to being, in some sense, most-correct.

It depends on context. A household of two earners working for big techs living in a $2M home in Bay Area is definitely comfortable, but by no means upper class. Most of their incomes are still from a W2 form and they still have to budget for kids' school, retirement, etc.