The PMode.Security.SendReceipt parameter is currently defined as: "The Boolean value of this parameter indicates whether a signed receipt (Receipt ebMS signal) containing a digest of the message must be sent back."

So setting the value to true will result in sending a signed Receipt. It is not clear if setting this parameter to false will result in sending an unsigned Receipt or no Receipt at all. It is more logical if this parameter indicates whether a Receipt should be sent and use other parameters to configure how to secure (sign/encrypt) that Receipt.