There isnt a clear line. But upper class generally meant having land, having a coat of arms, having a reasonably long descent from "genteel" people.. and if you had a profession, one of the genteel professions - law the military or the church. The Fergusons were landed gentry but they did not have much land, just a farm and were not that flush with cash wealth. Diana's family were titled, had a large house and an estate and a good deal of liquid wealth
In addition to their lavish, Grade I country seat and rural estate, the Spencers also own a large townhouse in London which, I believe, is let out on a long-term lease that provides a significant annual income.
Diana's father was an 8th Earl with a title created in the 18th century for a cadet great-grandson of a Duke. The family itself dates back to the 15th century and is associated with one dukedom and two earldoms, not counting lower titles. Many family members have been courtiers or in the service of the Crown for centuries. So maybe they are not quite as the Cecils or the Howards, but I would say the Spencers are probably in the upper echelon of British aristocracy and, although they are not the richest aristocrats either, they are a fairly wealthy family compared to other peers. In objective terms then, Lady Diana had a much higher social standing than Fergie or Camilla.
Still, the Spencers are "subjects" to use Lady Mountbatten's choice of words. Compared to a time when princes would only marry daughters of kings or sovereign dukes/ grand dukes, Diana was a downgrade then. But, as other posters have mentioned, the legal definition of a "dynastic marriage" is simply one whose descendants retain dynastic rights, i.e. a position in the line of succession to the Crown, and, in the United Kingdom, any marriage of a person in the line of succession that was consented to by the Sovereign was legally dynastic. There was no legal requirement of "equal marriages" as in some continental European monarchies.
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