The 6th generation of wireless mobile networks (6G) is envisioned to extend the capabilities of cellular communication networks into new dimensions, such as sensing, integration of artificial intelligence, and integration of other networking technologies. In addition, 6G is often described as the generation that will be the “Network of Networks” (NoN). One of the NoN building blocks is provisioning of local connectivity by so called 6G Sub-networks (SNs). Despite the fact that the term “Sub-networks for 6G” is already widely used, it still lacks a clear technical definition. This article aims to address this gap by providing contextualization and a description of the concept from various relevant perspectives such as legacy, autonomy, and topology. Moreover, a concept for an architecture is proposed, including Sub-networks, their interaction with associated user equipment, other Sub-networks and “Parent Networks” (PNs). These concepts are then mapped to a variety of use cases from relevant vertical domains, like vehicular Sub-networks, Sub-networks for collaborative and autonomous robots in factories, remote operation of aircrafts, airplane onboard systems and emergency services. Finally, a comparison of the described Sub-network concept with existing approaches in 3rd Generation Partnership Project (3GPP) is provided, stating similarities, differences, and future requirements for standardization.