I'm accessing the same VM from two different installations of OS X (10.11 and 10.12) on the same Mac. Each time I boot into the other system, I am asked by Fusion if I moved or copied the VM. Why, and which alternative should I choose?
You should select "Move".
The thing that happens when you select "Copied" is that the hardware of your VM gets a new set of ID's.
So for your operating system it is similar to changing the hardware, this will cause things like network addresses changing.
This is good when you have a copy as then you can start up 2 copies and access both of them from the network.
It is bad if you have a guest OS that is linked to the hardware such as Microsoft's OS's. Selecting copy might lead to having to re-activate the operating system.
As you want to use the same OS from within different host OS's, the normal action is "Move" as it doesn't change anything for your guest OS.
Just registered to share Something with you. Add:
uuid.action = "keep"
to the corresponding *.vmx File and Fusion/Workstation/ESXi
will never bring up the "moved or copied" Dialog for this VM again.