What is "call for nomination" for senior positions in university?
The potential candidates for this kind of high-level positions typically are people that already have a very busy job, and they do not actively search for jobs. It may be that the hiring process is more of the head hunting type rather than the advertisers passively waiting to get many applications and then filtering them. Of course, direct applications are allowed for the sake of openness and for not limiting unnecessarily the pool of candidates, but it may be the case that the ideal candidates would not apply uninvited. The search committee may not be aware of all the qualified people that might be interested of or suitable for the advertised position, so nominations are useful for them in order to bring new potential candidates to their attention. Once they spot a candidate, they may make them an attractive offer that is customized to the candidate. In the case that the nominator knows nominee from a distance, a direct hint by the nominator may not even attract the interest of the nominee to apply, while if the search committee finds the nominee interesting they may actively try to convince the nominee to apply through more elaborate methods than, e.g., a simple email that may not attract the attention of a very busy person.