That is correct. You have to get used to the AVR chip using a single pin for multiple uses. For example the Arduino's "analog input" pins can also be used as Digital I/O pins.
In-system Serial Programming uses the SPI interface:
Pin 16: SS: Slave Select (not used for ISP)
Pin 17: MOSI: Master-Output/Slave Input
Pin 18: MISO: Master Input/Slave Output
Pin 19: SCK: SPI Clock