The Slave Register is a website self described as:
The Slave Register (TSR) provides a central place for the registration of slaves and submissives. Each registration is assigned a unique Slave Registration Number ("SLRN"), and owned submissives and slaves can display an ownership certificate. You can create and maintain a registration here on the www.slaveregister.com website.
SLRN's are also frequently converted to bar codes as well and are sometimes applied as brands, tattoos, or placed on other paraphernalia available through the website's affiliates.