It is our tendency in English to, once we establish a pattern, cut the language down. Instead of saying, "I went to the store; I did not go to the bank," we often cut the second sentence down to a shorter version.
...I went to the store; I did not go to the bank.
...I went to the store, not the bank.
...She was always here on Mondays; she was seldom here on Fridays.
...She was always here on Mondays, seldom Fridays.
The result in these instances is called a "contrasting element." It usually begins with "not," "never," "seldom," or "rather" and always takes a comma or commas.
...I saw the doctor on Mondays, never Tuesdays.
...We socialized during the week, seldom on the weekends.
...She was not with her sister, rather her mother, when the accident took place.
Happy punctuating!
Margie