OnPrepareStarted called in main thread before the generate started. To load RAWs, textures, etc.
In MapMagic < 2.0 it's called once for each generator for each terrain chunk. In > 2.0 it's called once for each terrain chunk.
OnGenerateCompleted called in a generate thread after all of the generate work is done and terrain apply is queued. Called once for each chunk.
OnGenerateFailed (v2.0 only) called in a generate thread after MapMagic has encountered an error during generate and could not create chunk. Called once for each chunk in case of error only.
OnApplyCompleted called in main thread after the chunk is generated and generate results were applied. At the moment of OnApplyCompleted the chunk is ready and will not be changed by MapMagic until next generate. Called once for each chunk.