From United's Purser Bulletin (Aug/Sept):

"Recently, we have received several inquiries regarding where the Purser
works on the airplane. The Purser’s work position is pre-assigned by airplane type per the company standard.

Please refer to the Staffing and Service Flow Guide Sections of the Flight
Attendant Operations Manual (FAOM) for additional details.

The company standard for the Purser’s work position on all International and
North America Aircraft is as follows:

All narrow-body aircraft (except the B757): A319, A320, B737

• Purser works United First galley/aisle.

B757 aircraft

• When one flight attendant is scheduled to work UF, the Purser will work UF galley/aisle.
   
• When two flight attendants are scheduled to work UF, the Purser will work UF aisle.

All wide-body aircraft: B767, B777 (2-class and 3-class configurations), B747

• When one flight attendant is scheduled to work UF, the Purser will work UF galley/aisle.
   
• When two or more flight attendants are scheduled to work UF, the Purser will work UF aisle."