An isiduko (clan name) tracing descent from a specific founding ancestor — outranks a registered surname in social importance and is recited as the highest form of respect. Worn by Nelson Mandela, by whom it became known worldwide.
Source: Xhosa isiduko (clan-name) tradition, cross-referenced across Wikipedia (Xhosa clan names) and independent Xhosa-naming resources (southafricanclans.com)