Use 'bailiwick' to describe someone's area of expertise or authority, often within a job. Negotiating contracts is exclusively Robert's bailiwick.
Pro Tip 2/3
Modern Specialty
'Bailiwick' is used metaphorically today to refer to a person's special interest or skill area. Organic gardening is really Sarah's bailiwick.
Pro Tip 3/3
Not Physical Space
In modern usage, 'bailiwick' refers more to abstract areas than physical locations. Video game design is the programmer's bailiwick, not the physical office he works in.