Categorisation by occupation generates an occupations index (oi.html) along with a separate sub-index for each occupation (oi_<id>.html, where <id> is a unique 4 digit number).
Each occupation listed in the occupations index link to a per-occupation sub-index. Entries in the occupation sub-indexes are linked to individual vacancy pages.
Categorisation by occupation is not enabled by default, to enable vacancy occupation index generation check the Generate occupation indexes option in the Vacancy Publication Options dialog (Tools->Vacancy Publication->Publication Options).