Who hires a visiting professor? When a professor stays in another university as a visiting professor during his sabbatical leave, his salary is partially paid by his home university. Then, he is not a regular employee of host university, but how is the official processing?

For example, he needs lots of employment stuff such as ID, parking, etc. Thus, he should have a profile with the human resource.


Is he hired by the university (HR) or directly by a department?
Does he need to provide regular documents such as academic credentials (diploma, transcript) or an employment status from the home university is enough?
Following these questions, when he teaches a course or supervises a student, what is his employment reference in official records? I mean can a guest professor teach/supervise without official records in the university HR?


EXAMPLE: A full professor goes to another university on sabbatical leave whether for teaching or research. He needs ID card, and normally HR or similar central unit issues the ID card, not individual departments. Shouldn't he have a record in the HR files as the reference for the ID card issued?

My question is: When the HR is creating a file for a visiting professor/scholar/researcher, what documents should be supplied?
Let me clarify some terms:


Despite the language,  Visiting Faculty are not usually faculty who are gainfully employed at one university who are just staying at another university during their research leave. 
Most visiting faculty are "Visiting Assistant Professors" who are (usually recently graduated) scholars who do not yet have full-time employment and are hired as one-year (or two-year) replacements for professors who have gone on sabbaticals. This is different from an adjunct as the recipient university is hiring you full-time (not per course) for a term of one or two years.
Visiting (Full or Associate) Professors: Only rarely would a tenured, senior faculty use their sabbatical year to go to another university solely to teach. Exceptions would be: 1) they aren't being paid during their sabbatical year and need the money; 2) the recipient university has resources that they would like access to (archives, etc.); 3) they  want to live in the recipient city for a year, etc. This last one is the most common -- especially with American faculty visiting European capitals (sigh... Paris...).  Some faculty are contractually forbidden to use their paid sabbaticals in outside teaching -- at my university, a sabbatical is a leave from teaching responsibilities but it is ultimately a time when I am supposed to be doing research. 
There is also the category of Visiting Researcher - this is the category which most visiting full faculty would use on sabbatical at another institution if they had no teaching responsibilities. It would not normally come with pay or any benefits except library/archive/gallery access, e-mail, and an office. 


Thus if Prof. Smith@UniX goes on sabbatical, then Visiting Asst. Professor Jones@Unemployed is hired by UniX to replace Professor Smith.  

To answer your specific questions, UniX is entirely responsible for Prof. Jones' salary and benefits. 


Hiring is usually done by the department who needs a replacement. Often a full search is not run and sometimes the hiring is done on the Chair's prerogative. While HR of course handles the details after the department makes their choice, it is rarely HR that runs the search from the very beginning.
Credentials may not be checked as thoroughly since it is a temporary hire.
Visiting Faculty have appointments in their recipient universities and can serve as the instructors on record for courses. Depending on their term (and the rules of the institution) they can also serve as readers for senior essays, but usually they do not serve on doctoral dissertation committees. They are also usually exempt from most service work. 



Biographical note: After I received my PhD, I taught for a year as a visiting assistant professor, as a sabbatical replacement. This was before I received my first tenure-track job. [note: simplified version of a complex reality]