In the context of escort sex business COF means that either the escort or the client cums on the face of the other participant in the sex-act after vaginal anal or oral intercourse.
Besides a specific spot on the face or head where ejaculation is allowed, COF is also a kind of sexual perversion, the symbolic act of humiliation and super-ordination over woman. Of course, simply because of the dirty fantasy, mass of men crave after a woman or after an escort girl whose face they can cum on. This tendency can be observed on those generation socialized on porn, who are used to watching cum on face as the final act of porn shows. And if a girlfriend or wife does not allow it, then escort girls will do the job whose favourites include COF. Then it's just a successful date (escort meeting), and after winning the sympathy, there can come anal sex or the long-awaited COF, ejaculation on the face.
Cumming on the face or into the mouth – as it has been mentioned previously – can make one feel humiliated, subordinated or excessively dominated, so many escorts tend to undertake only the fourth member of the group C: COB(coming soon)