what abreviation rule or rules does the term "UX" follow?

I am intrigued by the use of the letter "x" in the abbreviation of the term "User Experience" does this follow any specific language or phonetic rules?

As Hot Licks points out in a comment, several posters provide background on the use of UX as a short form of "user experience" in response to the question Is 'UX' as an abbreviation for 'user experience' meant to be ironic? posted on the Stack Exchange's own User Experience site. The upshot of the comments is that X commonly appears as a single-letter abbreviation for whole words that begin with the letters ex.

One of the earliest such uses may be in connection with the short form PX. The X in this initialism commonly stands for exchange, although the P in PX may stand for private in some cases or for post (or postal) in others. For example, from Western Electric Electrical Supply Year Book 1918 (1918):

Private Exchange (P.X.) and Private Branch Exchange (P.B.X.) switchboards , such as our sectional unit type, range in capacities from 20 to 80 lines and over. P.X. and P.B.X. boards are also made up in either desk or cabinet style, ranging in capacity from 60 to 600 lines.

From Federal Security Agency, Civilian Conservation Corps, "Camp Life Arithmetic Workbook 5" (April 1941):

The Post Exchange performs a service for the CCC which merchants perform for civil communities. Through the P. X. the company serves its members in the capacity of a merchant.

From "Applause for Major Hastey," in Air Corps News Letter (April 15, 1936):

Probably all of us walk in and out of the Post Exchange more frequently than in any other building on the post. Our PX is still undergoing a length and breadth overhauling that is resulting in a store as modern as any PX on any post.

An earlier instance, in The Leatherneck (July 1932) identifies the checkers champion at a post in Nevada as "Pfc. L. Leslie, PX book-keeper," indicating that PX was already in use in U.S. military jargon at that date.

Another early use of X to stand for a whole word beginning with the letters ex is in connection with clothing sizes, where the X stands for extra, as in XS (for "extra small") and XL (for "extra large") XL for "extra large." For example, from an advertisement for Mead's Scarsdale in the Scarsdale [New York] Inquirer (August 18, 1944):

SIZES S, M, L, AND XL IN UNDERSHIRTS 55¢ and 1.00

Somewhat older than that instance are instances from Australia of the short forms XSSW (for "extra slim small woman") and XOS (for "extra out sized"). For example, from an advertisement for John's Silk Store in the Newcastle [New South Wales] Morning Herald and Miners' Advocate (June 16, 1932):

Face Cloth Coats. Of superior quality, in ultra-smart styles, featuring inset backs of reversed material. Smart collars of large Coney fur. Colours include Brown, Navy, and Black. Sizes include X.S.S.W. to X.O.S.

As Roger Attrill details in his answer to the User Experience Stack Exchange question noted at the beginning of this answer, X has also seen service as a single-letter abbreviation for executive (in the initialism AX, short for "analyst executive"), experience (for example, in the initialism VX, short for "virtual experience), and extremity (in the initialism LX, short for "lower extremity"). X also shows up as a whole-word abbreviation in X-Games (where it stands for extreme), XFG (short for "exfoliation glaucoma"), XGA (short for "extended graphics array"), and XML (short for "extensible markup language").

Why did this seemingly odd tendency to use X rather than E as the signifying letter for certain ex words in initialisms arise in the first place? The answer may simply be that the relative rarity of words that begin with x recommended X as an alternative to E in order to reduce the likelihood of ambiguity. Compare Wikipedia's list of acronyms that begin with X to Wikipedia's list of acronyms that begin with E, and you can see the appeal of using a relatively infrequent starting letter.

Conclusions

The use of X in the initialism UX to stand for experience does not follow an ironclad rule for dealing with words that begin with ex. For example, the acronym ELF is sometimes used as a short form for "extremely low frequency" and for "extraterrestrial life form"—although there is no competing widespread initialism XLF that might muddy the waters if that spelling were used for one of the terms instead.

Nevertheless, a considerable number of words beginning with ex are represented by the single letter X in various initialisms and other abbreviations. This practice has been around for more than a century, as the 1918 example containing PX and PBX (as short forms for "private exchange" and "private branch exchange, respectively) noted above confirms.

With regard to shortenings of the word experience to X, User Experience site participant Roger Attrill notes—in addition to UX for "user experience"—the kindred forms CX ("customer experience") and VX ("virtual experience").

There may be no formal rule at work here, but there is certainly a widespread (albeit sporadic) practice across multiple words beginning with ex to reduce them in initialisms to the single letter X.