Always ask for a transfer/fare receipt, even if you are not going to transfer. Transfer/fare receipts are valid for 90 minutes.
sfmuni.com has a text description of the 1-line
1: Alternate service available
For weekday California Street Express Service, See 1AX and 1BX schedules.
Buses and F-line: Always ask for a transfer, even if you are not going to transfer to another line.