Minecraft wiki entry for Updating_old_terrain, associated text: Aggressive mode deletes chunks that contain any of the selected biomes. For example, if you decide to delete all river biomes in aggressive mode, the plugin will delete a chunk as soon as it contains a single block of river biome, no matter which other biomes are present in that chunk.
Cautious mode only deletes chunks that contain nothing but the selected biomes. For example, if you tell the plugin to delete ocean and beach biomes, then it will only delete chunks in which each block is part of an ocean biome or a beach biome. If there is a single block in that chunk that is neither in an ocean nor in a beach biome, then it won't delete that chunk.