Or, "have their main jobs at the university".
It is mentioned because it's not uncommon that professors have their main money-earning jobs elsewhere and have some other motives for working additionally at a university.

Compare:

Norway
[..] Professors who only work part-time, typically 20 %, and who usually have a different main job (for instance as a Consultant at a university hospital), are called Professor II, meaning this is a secondary job, but they need to have the same competence as other Professors and are styled as simply Professor.
http://en.wikipedia.org/wiki/Professor#Norway


Also referred to as "primary employment"...
http://www.communities.gov.uk/documents/fire/pdf/151255.pdf
http://www.econ.yale.edu/growth_pdf/cdp781.pdf