ShropshireMiss
The publication route is normally a higher doctorate rather than PhD and is for someone already with a distinguished record of publication.
I may not be correct in this but it may be that higher doctorates (DSc and DLitt) were once the only degrees that could be awarded by the prior publications route yet that that was later extended to also there being the possibility of obtaining a PhD by a prior publications route as well.
I expect that most PhDs are awarded as the result of supervised research that takes place after registration for the degree, registration requiring being offered a registration after having applied.
Yet where someone has, for example, worked in an industrial research laboratory and has published papers in learned journals over a number of years, a PhD by previously published work can sometimes be awarded, after registration for a shorter period after submitting the publications together with a thesis-like document that reviews the publications that is examined and passed and by passing a viva voce examination.
Yet publications resulting from working in an industrial research laboratory is just one possibility, just mentioned here as an example.