The application code is a unique identifier. It's based on an algorithm of hardware and software and each instance will have a different application code to unlock the license application on your server. There are some things such as IP address, server name, and machine ID that will affect the identification.
The "MachineID" is created based on a number of specific variables about your system in order to generate a unique ID. Unfortunately this is not something that we create, but is rather something that Microsoft creates that is commonly used in licensing because it is always unique. Microsoft is pretty tight lipped about how exactly this MachineID is created, but any major changes to hardware will trigger a change in the MachineID.
A change in Network adapters for example would be likely to cause a change in the MachineID (including virtual adapters). A change in the motherboard being used would also be likely to cause a change in the MachineID. But something more commonly changed like a HardDrive, or RAM would not likely cause a change in the MachineID. Changing CPU Cores in a VM for example should not change the MachineID, however, if you were to actually change the physical process, I am unsure if this would cause a change or not. Changing things like the Computer Name, or Windows ProductID are also likely to change the MachineID.
- If you are making ANY changes to your NIC's, virtual or physical, I would suggest deactivating the licenses, and reactivating them after the change is complete.
- If you are moving the VM from one physical machine to another, I would suggest deactivating the licenses, and reactivating them after the move is complete. If you are moving them to a new location on the same machine, this should not be necessary.
- If you are changing any information typically used to identify your PC on the network (ie Computer Name), then I suggest deactivating and re-activating your license.
Unfortunately I do not have a specific list you can use for every item that may impact the MachineID as Microsoft simply doesn't provide that kind of detail about how this unique identifier is created. But the above should deal with most of the common areas that might cause this unique identifier to change.