UL certificates are in fact NRTL (Nationally Recognized Testing Laboratory) safety certificates that are issued by Underwriters Laboratories, UL LLC based on the following safety standard:

The UL safety certificate is one of few names for the NRTL safety certificate which can be issued by other accredited agencies like TUV Rheinland, Underwriters Laboratories, UL LLC  and CSA Group (for example) recognized by OSHA (the United States Occupational Safety and Health Administration). If you need such a certificate you could log in a ticket with us by writing an email to techregshelp@hp.com.