Hello Alexis
When you configure a network device, such as the Core Router in this lesson, to operate as an NTP server, you don’t actually have to explicitly configure it as a server. The fact that the switches are configured with the command ntp server 192.168.123.3 makes the Core Router the NTP server, and the switches the NTP clients.
The ntp server pool.ntp.org command configured on the Core Router actually makes this device an NTP client to the pool.ntp.org server, but at the same time it retains the role of NTP server for the switches.
Now the master command is simply used to let a device know to consider its own clock as valid, and it is also used to manually set a stratum number for this NTP source. This way, clients of this device will adopt their stratum number based on the locally configured one. This type of configuration means that no external NTP sources are used for synchronization.
I hope this has been helpful!
Laz