When the soot loading in the filter reaches its limit the vehicle's ECU will initiate post combustion fuel injection to increase the exhaust temperature and trigger regeneration. If the journey is too short while the regeneration is in progress, it may not complete and the warning light will come on to show that the filter is partially blocked.
Forced regeneration is required where soot levels have increased within the DPF to a point where normal regeneration cannot be performed. At this point, the vehicle will enter a 'restricted performance' mode. If left the soot loading will keep rising.
At this level, diagnostic tools must be used to force regeneration. At around 85% soot loading, regeneration can no longer be performed on the vehicle and the DPF will need removing to be cleaned or replaced.
It is possible to remove the DPF and have the ECU reprogrammed so the vehicle can run correctly without the filter in the exhaust, it is however not possible to remove the filter without reprogramming the ECU.