VERY IMPORTANT!
The PLCs are essentially temporary backups and exist only so you can save without having to send and receive, and so you can continue to work when disconnected from the network.
They can be used to create alternates by saving variations with the same sign in. In this case it would be appropriate to (temporarily) link the LBK to each PLC in turn to produce the alternate sets. This is a special case though, and the links should otherwise always be to the PLP.