Unique Job Postings denotes the number of de-duplicated job advertisements listed by different companies on career sites and job boards.
De-duplication is a process job postings must go through in order to best approximate demand for certain jobs. The reason this process is so important is because employers can list one job posting on many different career sites and job boards. By itself, this non-deduplicated postings number by itself is a dramatic overestimate of actual job demand.
Read our article for a full discussion of issues surrounding job postings and traditional labor market information.