Usually, this was a simple bureaucratic process. You called support, gave them the serial number, they sent a file ending in .LIC , you applied it, and went back to sleep.
For further technical details, you can consult the official IBM Support documentation on generating FoD keys. Ibm Imm Activation Key
An activation key for IMM2 on an x3650 M4 will NOT work on an older IMM1-based x3650 M2. Always verify your IMM firmware version before purchasing or downloading a key. Usually, this was a simple bureaucratic process