The content is - "This is a signed cheque by operator, regarding a promise between the end-user and the operator. When the op couldn't include corresponding Tx within certain blockheight of childchain, the op must willingly pay back the same amount of fund from L1 collateralized op's fund to the holder of this cheque."
The op gives a signature for only the holders of FF NFT.
FF NFTs are sold on Merchant Wallet. This is a open source Plasma wallet variant. Everyone can be the holder of the FF NFT, and be the acceptee of FF Tx.