It depends on where you go. Some universities enable exchange students to take courses in different fields (in different departments of faculties), while others require exchange students to register in a specific department and you can only take courses in that department. This information can usually be found in each university’s fact sheet. There are links to the fact sheets in the list of universities that appears in the Call for Applications.
It is important to check in advance whether it is possible to take courses in different fields in order to build your course load correctly, and also in order to adapt your academic program at Hebrew University before and after the student exchange semester, if necessary, with the help and approval of your faculty’s student exchange advisor. For example, if you go abroad to a university which allows you only to take courses in a single field, you may need to take more courses in your second field before or after you go abroad.