When you set up a synchronization system you must decide which unit is the master. All other devices are then slaved to this unit, which means they will adjust their playback speed to the master’s.
When a synchronization signal is coming in to Cubase VST, from another device (such as a tape recorder, a disk based recording system, a drum machine, another sequencer etc), this device is the master and Cubase VST is the slave. Cubase VST will adjust its playback to the other device.
When you set up Cubase VST to transmit synchronization signals to other devices, Cubase VST is the master and the other devices are the slaves; they will adjust their playback to Cubase VST.
Cubase VST is a very capable synchronizing device. It can operate as both a master and a slave at the same time. For example, Cubase VST might be slaved to a tape recorder transmitting time code, while at the same time transmitting MIDI Clock to a drum machine, acting as a master for that.