Please provide a definition of this concept, which does not appear in Michel or Yvert catalogues. Specific UK??
A regular stamp is thus not ipso facto definitive. The first non-definitive, Commemorative stamps were issued in the end of the 19th century: Peru in 1871, New South Wales in 1888 (considered the first issue with explicit mention of a Commemorative issue).
I see that some add Definitive for any regular issue in ths Wiki. Of course, by default, an issue is regular; if not, additional info is given. The added value of this strange English cat (definitive means in normal language without end, as opposed to an essay) is thus very limited. There is no need to change the page names to add this!
I have proposed a definition for Austria-Empire. I object the relevance of Definitives for overprints, surcharges etc. Rie puts here any regular, as if a by default parameter was of any interest.
A category should have added value!