‘Rx’ is an abbreviation of “Received” or “Received transmission” when used in the context of fax machines and office equipment.
The Merriam-Webster dictionary defines “Rx” as a “prescription.” The Oxford dictionaries define Rx as “a doctor’s prescription.” Both of these definitions are the more popular use of “Rx,” but do not apply to the context it is used in for fax transmissions.
In the context of fax, it is often used for reporting and logging. “Rx reports” refer to “received fax reports.”
History of the term
The format of the abbreviation is likely derived from “tx” (an abbreviation of ‘transmission’).
The most popular use of “Rx” is in medicine as an abbreviation for “prescription.” The “Rx” stands for “take,” literally an abbreviation of the Latin word “recipe” which means “take.” The symbol ℞ is often used in this context.
Wikipedia contains a list of other uses of the term “Rx.”