http://www.minecraftforge.net/forum/topic/18578-solved172what-exactly-is-mask-argument-of-setmutexbits-in-ai-tasks/

Using these categories:

• Uncategorised, compatible with every task
• "interactive" AI, including attack: this.setMutexBits(3);
• "movement" AI, this.setMutexBits(1);