"Requested department" is a special field that can be used within application forms once you've defined one or more requested department options. (From Setup select Application forms then the Requested departments link). Unlike most fields, this field can be used to filter application forms because it's the same field used on multiple forms.
Requested department does not affect the applicant and isn't stored in her profile: the value is only stored in the application itself.
A person can have multiple requested departments on different applications.