The Lisa CPU board has a PROM which is used primarily in a state machine to generate the video signals (vertical/horizontal retrace and composite sync). This PROM also contains two serial numbers. An AppleNet ID and a serial number for the Lisa. These numbers are not directly accessible, but retrieved bitwise from the video data stream during a retrace.
The serial number is (as far as we know) only used by the Lisa Office System to prevent the copying of the Tools (applications such as LisaWrite, LisaCalc, etc.)
As soon as you copy one of the Tools to a hard drive, the Lisa writes its serial number on the Tool (on both the floppy and the hard disk.) From this point on, that Tool can only be used with that Lisa.
Also, the first installation diskette for Lisa Office System stores a copy of the serial number to prevent its use on another Lisa.
David Craig has a Tool deserialization paper that talks about removing the serial number available here: http://www.cs.dartmouth.edu/~woz/deserial/ and here: http://lisa.sunder.net/cgi-bin/bookview2.cgi?zoom=-3?image=0?book=7
http://www.kreativekorp.com/kko/software/lisadisk.html claims to have a tool to that removes serialization from Lisa floppies.[LisaFAQ] > [Lisa Software] > [Lisa Office System] (Comment on this answer)