Plot public key
So, what is the plot public key shown in the above image? The plot public key is a 2/2 BLS aggregate public key, which is generated by combining the farmer public key with the local public key. The local public key is a totally random key that is created for each plot, and put into the plot. They are not children of any BLS keys, and therefore totally independent between plots. The farmer key is a key that the farmer machine stores.
Each block requires a signature from the plot key. What this means, is that we need to combine two signatures: one from the local secret key, which is generated by the harvester, and one from the farmer sk, which is generated by the farmer. The farmer combines both signatures to generate the plot signature.
это с офф сайта