and basically it meant something targeted at or interesting to only those with special knowledge or interests.
People with special knowledge or interests are specialists, so the word you're looking for should be specialistic.
Definition of specialist from Merriam-Webster on line
one who specializes in a particular occupation, practice, or branch of learning
an enlisted rank in the U.S. Army corresponding to the grade of corporal; also : any of several former enlisted ranks corresponding
to the grades of sergeant through sergeant major
an organism (as a bird) specialized especially in food or habitat
Specialistic, from Dictionary.com:
- pertaining to or characteristic of specialists or specialism.
Origin of specialistic
1880-85; specialist + -ic