‘Tx’ is an abbreviation of “transmission.” The term is frequently used with fax machines and fax transmissions.
In the context of fax, it is often used for reporting and logging. “Tx reports” refer to “transmission reports” and “Tx status” refers to “transmission status.”
In medical terminology, ‘TX’ can mean a transplant, treatment, therapy, and a number of other meanings.
In linguistics, ‘Tx’ is used as a digraph for Basque, Catalan, Fataluku, and some indigenous languages of South America.
Wikipedia keeps an extended list of other uses of the term ‘TX.’