It's a communication occuring in the blockchain for bot and master or hacker and colluding party which provides commands for direction or execution. If they change the wording of their communication, good luck finding them again.
Now that you say it I have the killer application where to use Bitcoin and the blockchain.
Botnet C&C. Passing botnet commands trough IRC and Twitter is so last century when compared to passing those same commands using Bitcoin transactions.
Now I just hope no real bot herder reads this and decides to implement it. I will want a cut of the profits lol