Hello @sgabran and welcome to ST Community.
The difference between the two registers is that "REG_LR_PKTRSSIVALUE" captures the RSSI value of the last received packet, while "REG_LR_RSSIVALUE" captures the current RSSI value of the receiver.
"REG_LR_PKTRSSIVALUE" is updated only when a packet is received, and it holds the RSSI value of that packet. On the other hand, "REG_LR_RSSIVALUE" is continuously updated with the current RSSI value, regardless of whether a packet is being received or not.
So, if you want to get the RSSI value of the last received packet, you should use "REG_LR_PKTRSSIVALUE". If you want to get the current RSSI value of the receiver, you should use "REG_LR_RSSIVALUE".
I hope this is helpful for you.
Issam