This is what the manual says;
Internal: The uTrack 24 will run off its own internal sample clock; you are free to select from four different sample rates to run the internal clock at. When running from the internal clock, the settings bar will display “Int” to reflect this internal clock setting.
Wordclock: The uTrack 24 will slave to an external word clock signal that is connected to the “word clock in” BNC connector on the rear panel.
When using an external clock, the settings bar will display “Ext” to reflect this setting. However, If you have selected external clock in the settings menu, but no valid external clock is actually connected to the word clock input connector, the “Ext” label will flash on and off, to reflect that the clock signal is missing and no recording will be allowed to take place. If you try to record when the unit is set to external clock, but no clock source is connected, no recording will take place. Instead the following message will temporarily appear