How front office generates revenue?

What are the ways to generate revenue in the hotel front office?

The major revenue generation by front desk department is by the sales of Room. other than this revenue are generated by telephone calls, travel desk, bell desk or concierge services like stamp, postcard sales etc.

I think other than room sales and travel desk front desk earn from Foreign Currency Exchange commission, Cash Paid out to guest on credit card where hotel normally add a commission, other referral sales commission on ticket booking, tour packages booking, boating bookings, Business centre room space, secretarial services and computer usage etc.