Purpose of PPP field in nRF24L01+ W_ACK_PAYLOAD command?

[laneh]

In the mRF24L01+:

Unlike MultiCeiver mode, I have one TRX and multiple PRX units sending back ACK payloads.

In the PRX, what is the purpose of the PPP field in the W_ACK_PAYLOAD command? If the PRX returns an ACK packet using the same address it responded to, why would a pipe number matter?  Is the PPP field used only for MultiCeiver systems?

[laneh]

Thank you for the missed-broadcast warning. I handle this by including a "broadcast not received" value in the ACK packet data. Whenever the receiver dispatches an ACK packet in response to the transmitter broadcast message, it loads this "invalid" code into the ACK packet, and when a broadcast message is received the receiver flushes the TXFIFO and loads an updated, valid value. When the transmitter receives the "invalid" indication it knows to try that receiver again.