to be one's department
01
to be responsible for something, showing a sense of authority in that area
例
Is n't handling customer inquiries your department, Chris?
Coordinating project timelines is Alex's department in the project management team.
02
to know a lot about something, indicating a sense of expertise in the matter
例
I can look into it, but legal matters are not exactly my department.
I'll do my best, but graphic design is n't really my department.
