I have the1997 edition of the SMC's Munro's tables which lists the Furths on a single page between the list of Munroists and the Corbetts. It lists 6 in England, 15 in Wales and 13 in Ireland. The preamble states "The exact number of summits is open to debate but all the controversial tops would normally br traversed as part of the ascent of the principal peaks. A complete ascent of this list... is an achievement recognised by the Clerk of the List"
I guess that to be recognised as an acheivement by the SMC, the SMC list of Furths should be used.
Another use of the term is within my university as we occasionally have to consider for approval "study furth of the university" by research students.