|NTP server:|| (DNS, IPv4 or IPv6 address)|
When NTP server is not set, this device still can accept time messages via LAN broadcasts. If you have local NTP server in your LAN that broadcasts time messages, then it's not required to set NTP server explicitly.
When NTP server is set, then it's used exclusively and broadcast messages are ignored. Choose trusted NTP server that is topologically close to you (preferrably in your network or your upstream ISP).