System Builder or OEM is locked to the motherboard of the 1st system it is installed on.
ie. not meant to be transferred from PC to PC. Also the vendor of the PC with the OEM licence is responsible for for the software (Advice etc.) The retail version is more flexible, can be installed on any machine as long as only one at a time. Microsoft provide the backup for the software. OEM software can be upgraded.
Definitive answer regarding OEMhttp://prntscr.com/9537y