The debug section has some bit hackery tools for fast computation which make it hard to read.
Essentially N increases the speed of the clock, forcing a rollover not every 49 days, but every 49/(2^N) days. Offset also allows you to change the start value of your clock so the first rollover happens in say 1 minute instead of waiting a full 49/(2^N) days.
You would use this to test how your own code (which uses the T_PLC_MS()) functions at the rollover. For example do you have code that says:
IF input1 AND NOT AlreadySeenInput1 THEN
StoredTime := T_PLC_MS();
AlreadySeenInput1 := TRUE;
IF ( T_PLC_MS() > (StoredTime + 30000) ) THEN
ThirtySecondsLater := TRUE;
If StoredTime was 10000ms less than the rollover,
(StoredTime + 30000) would then be only 20000, and ThirtySecondsLater would come on immediately.
You could test that scenario in simulation in a matter of minutes with the debug code, just don't leave it on the real machine!
Warm welcome to all things Codesys!